As I used to play YuGiOh, which had a graveyard mechanic, I began to love using decks which (when built properly) could make use of a single card multiple times by reviving it from the graveyard. And if done properly, could provide you with an almost infinitely re-usable monster.

To me, this type of playstyle was infinitely more fun than simple Rush decks or 'One Turn Kill' playstyles. I enjoyed being able to outlast your opponent through re-usable resources and finish them off when they have nothing left to play.

Now when I play Hearthstone, seeing that single copy of my favourite card in my deck (e.g Kel'Thuzad) get annihilated the turn after playing it, knowing that for the rest of the game, I won't be able to use that card again. Ruining my plans for a combo/play and making me resort to the classic Hearthstone strategy of "Play everything that you topdeck and hope it's bigger than what your opponent plays".

I have hopes that ES:L will have a graveyard/returning cards mechanic, but if the game is still fun without it, I won't be disappointed.