Any additions from other players to the list below are welcome!
(also posted this in the official Bethesda forums: )

Deck Building

You are allowed to have 3 copies of a card in your deck (no matter of its rarity).
Only exceptions are Unique cards which are limited to 1 copy a card.

You recognize Unique cards by the additional “yellowish” gem in the middle of the top of the card.
e.g. Odahviing (added by SauceAlfredo) [28.08.2016]

Gameplay

• Creatures recently played to a shadow lane can be targeted with Actions and Summon effects of creatures while they have Cover.
• You can Silence a creature with Cover, removing the Cover effect and allowing you to attack the creature.
• A creature with “Summon: Give a creature ” can target itself with the effect.
  So a Barded Guar played on an empty board can at least give itself Guard.
• Speaking of Barded Guar, this is a pretty versatile creature.
  1. As stated above it can give itself Guard.
  2. It obviously can give one of your other (bigger/deadlier) creatures Guard.
  3. It can also give Guard to an “annoying” creature of your opponent that is “hiding” behind a creature with Guard. Allowing you to attack the “annoying” creature.
  4. And lastly it can be used to break Cover of a creature your opponent just played into a shadow lane, allowing you to attack it.

• You can Silence your own creatures to remove let´s say a Shackle effect, allowing you to attack with it (But be warned: ANY other effects on the creature will be Silenced too).
• You can target yourself with a direct damage spell (like Lightning Bolt). This might be some pretty rare kind of a desperate last ditch afford to break one of your own Runes to draw a card to let´s say get a Silence Effect to Silence a high health opponent creature with Guard to allow your creature(s) to finish off your opponent (which with the creature with Guard in the way would not be possible).
• Your hand limit is 10 cards.
• Unless you do not want to break (at least) one of your opponents Runes you might want to attack your opponent with all your creatures in a lane before playing an Action / Summon effect that deals damage to ALL creatures in a lane this way also killing your own creatures (of course consider to damage opponent creatures properly [if necessary] so they get killed by the damage your AOE effect deals to them).
• You can kill your own freshly cast Brutal Ashlander with a Firebolt or a Summon effect that deals 1 damage to trigger its Last Gasp effect of dealing 3 damage to a random enemy. You MIGHT hit something you could not hit otherwise, or damage something enough to kill it off with an attack of another creature (this again is a slightly desperate action).

• Basic order of attacking your opponent:
  Let´s say your opponent is at 8 health and you can attack him with a creature with 6 attack and a creature with 2 attack.
  You want to attack with the 2 attack creature first to bring your opponents health to 6 and then finish him off with your 6 attack creature.
  If you do it the other way round you bring your opponent to 2 health thus breaking his 5 health Rune. Giving him the opportunity to draw a Prophecy and so probably allow him to play a creature with Guard to protect himself, or to Shackle your remaining creature or even kill it.
• Forsworn Guide is best used on a (cheap) creature with a strong Summon effect (like Shrieking Harpy or Wardcrafter) OR on a high value damaged creature to (kind of) heal it (like a Storm Atronach without a Ward and at 1 health).
• You can sacrifice/kill one of your own creature in a full lane by playing another creature into the full lane. You then will be asked to select one of the existing
  creatures in the lane to sacrifice. This way you can trigger the Last Gasp effect of the creature (of course after attacking with it; House Kinsman comes into mind) and/or get it into your discard pile (there are cards that benefit from this) and/or to “activate” an effect that says something like “If a creature died this turn” (like Feasting Vulture or Disciple of Namira). (added by johnwhitegm in official Bethesda forums)
• The maximum number of creatures in a “normal lane” is 4 (Filed Lane and Shadow Lane in VS Play). In Solo Play sometimes there are smaller lanes. There the maximum number of creatures is 2. Whenever there is a small lane there always will be a bigger lane. There the maximum number of creatures is 6. (added by MacDermot through https://legends-decks.com)
• You can see the special effect a lane has by hovering your mouse over the small grey circle in the middle of the sideline. (added by MacDermot through https://legends-decks.com)
• Transforming a creature (with Mummify or Wabbajack) will not kill it. This way Last Gasp on a transformed creature will not trigger.
• A Creature with Guard never can get Cover.
• If your opponent is at 3 or less health and bings you straight down from 6+ health to 0 to -2 health it looks like you will draw a card for him breaking the Rune. If this card is Ransack you can play the card an win the game this way. (added by tjclark24 through https://legends-decks.com)
• Every Turn from 1 to 12 your max. Magicka is increased by 1. There are Cards that can increase your Magicka permanently to get more than 12 (like Tree MInder).

• If a creature with Ward has an effect that triggers if it loses the Ward (like Breton Conjurer) you can activate the effect again IF you manage to give the creature an new Ward (with cards like Wardcrafter or Lesser Ward) (added by Eirichur in official Bethesda forums).
• The 3-Win rewards are capped at 30 wins a day. So you can “collect” the 3-Win rewards 10 times a day. (reported by Julien7798 in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4w2hrz/whats_the_gold_limit_in_vs_battle_per_day/ )
• Creatures that can deal damage to other creatures by not attacking other creatures (like Dres Tormentor) will immediately kill a creature they damage IF they somehow get Lethal (by cards like Skooma Racketeer or Daedric Dagger). (added by KyteP1 in the official Bethesda forums)
• If you play a creature into the Shadow Lane and right after playing it (when it still has Cover) move it to the Field Lane (with cards like Dune Smuggler), the moved creature keeps Cover. (added by flossy in the official Bethesda forums)
• Support cards stack.
• You can’t Silence the effect on a creature generated by an ongoing Support card.
• The maximum number of supports a player may have at one time is 6. Playing additional support cards will require the player to sacrifice one to make room for the new support card. (added byjurassicahttps://community.bethesda.net/thread/70220 )

• Once a card is stolen, it is considered to be owned by the player ‘stealing’ it until the end of the match. Any interactions that cause the card to be moved back to the owner’s hand or to a discard pile, will occur for the controlling player. (added byjurassicahttps://community.bethesda.net/thread/70220 )

• If you have to draw a card and your deck is empty, you break one of your Runes. If you have no remaining Runes, you lose the game. (added byflossy in official Bethesda forums)

• A creature that gets it´s stats debuffed (by cards like Curse) will be considered wounded. Cards that have an effect like or will work on them . (added by @jurassica https://community.bethesda.net/thread/70220 )

• If you move a creature played into the Field Lane (with cards like Dune Smuggler) to the Shadow Lane, the moved creature gets Cover. (added by appsecit in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4wkbgn/interesting_mechanics_tips/ )

• When you have a creature in play that gives a newly summoned creature a buff (like Bruma Armorer) it will work on creatures that get summoned by other creatures (like Breton Conjurer). (added by appsecit in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4wkbgn/interesting_mechanics_tips/ )

• If you Silence a damaged buffed up creature. The damage will be silenced off too and the silenced creature will have the statline as if freshly summoned. (added by appsecit in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4wkbgn/interesting_mechanics_tips/ )
  Right now any damage will not be silenced off, the creature will just stay on it´s current health level (added by @Baugur over at official Bethesda forums 27.09.2016)

• If a creature summons a new creature at the start of your turn and if the lane of the summoning creature is full, the newly summoned creature will be summoned to the other lane (added by appsecit in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4wkbgn/interesting_mechanics_tips/ )

• You can use Cast Out on one of your own creatures. One useful target I recently came up with is any creature with Charge.

• When you summon Dres Renegade and you have shackeled creatures in play, those will be “unshackeled”. (added by MusicDroid7 in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4wkbgn/interesting_mechanics_tips/ )

• When a card requires a certain condition to trigger a “bonus effect” (like Golden Saint) the card will be highlighted with a glow effect on the left and top border of the card IF the requirement is met.

• If you steal a Ward from a creature (with a card like Ahnassi) this will not trigger the effect of a creature that says “losing a Ward”. (added by JHoward_FR in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4wkbgn/interesting_mechanics_tips/ ) [11.08.2016]
  Right now if you steal a Ward (with a card like Ahnassi) the “losing a Ward” effect will trigger! (updated 27.09.2016)
• If a creature with Drain attacks a creature with a Ward, you do no damage and thus do NOT Drain. (added by Gamadeus in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4wkbgn/interesting_mechanics_tips/ ) [11.08.2016]

• If a creature with Breakthrough does attack a creature with a Ward, the Ward will “suck up” all damage done, thus no damage to the enemy player will be done. (added by BattleOoze1981 in this reddit thread https://www.reddit.com/r/elderscrollslegends/comments/4wkbgn/interesting_mechanics_tips/ ) [11.08.2016]

• If you lower the health of a creature with Ward to 0 without attacking it (with a card like Murkwater Witch) and the creature with Ward has an effect “if loses a Ward (like Breton Conjurer), the creature is killed without breaking the Ward and thus the effect does NOT trigger. (added by Bylandt11 in official Bethesda forums) [11.08.2016]

• Pilfer ONLY triggers if you attack the enemy player. It does not trigger if you attack an enemy creature UNLESS you manage to give your creature with Pilfer Breakthrough and at least 1 damage ist dealt to the opponent directly. [12.08.2016]

• If you summon one (or more) creatures by either an Action or a Summon effect of another creature and a lane does not have enough space for the creatures summoned this way you will NOT be asked to choose an existing creature to sacrifice. The summon effect will just not happen. (added by @MorHawk in official Bethesda forums )[13.08.2016]

• If you DESTROY a Warded creature (with a card like Piercing Javelin) the Ward does not break and thus any effects of will NOT trigger. (added by @ashes52 in official Bethesda forums) [16.08.2016]

• Any ability of a creature that triggers if the creature deals damage will trigger if the creature deals damage in ANY possible way (including damage of effects from creatures like Dres Tormentor or Lillandril Hexmage) (added by @TalosLovesYou in official Bethesda forums) [12.09.2016]

• If the attack strength of a creature with Lethal is reduced to 0 the Lethal effect no longer works, because the creature has to deal damage in order to trigger the effect. (added by Nihlus_BRaga in official Bethesda forums) [13.10.2016]

• If you kill your opponent with a creature that has Pilfer, the effect will not trigger. Has no impact on gameplay, but on Quests. (added [17.11.2016] )
• You can use cards that deal damage when Summoned (like Ravenous Crocodile) on your cards that trigger an effect if they lose a Ward (added by White Bear 84 [09.12.2016] )
  

Quests
Removed this part because it got it´s own Article right here: The Quest system

Tools / Resources

Deckbuilder, Card Database
Well, this great site…

Full Progress and Collection Tracker:
https://community.bethesda.net/thread/64891?q=captn

Game Tracker by @JStarrG
https://community.bethesda.net/thread/65199

Collection manager @Faid
https://community.bethesda.net/thread/70111

Very good writeup about CCG´s in general and how they come into play in Elder Scrolls: Legends by @DYLVG
https://community.bethesda.net/thread/91081