Tavern Brawl is so FUN!
The single biggest thing that has pulled me back in is the new Tavern Brawl mode. It is a mode that is open from Wednesday to Sunday each week and it has a rotating rule set. Every week is a new and crazy set of parameters. As soon as you win your first Tavern Brawl game for the week you instantly earn a card pack.
All of this together totally appeals to me.
I highly recommend checking out Tavern Brawl if you've even played Hearthstone. The rulesets are so fun and varied. This is the most fun I've ever had with the game.