I haven't been playing Hearthstone for a long time as I was bored of it after a while. I did have fun though and the upcoming expansion brings me back to the game again as during the launch event, which should last roughly a month or so, the first wing (chapter) is free during this period only!
I'll also need to start grinding out more in-game gold to buy the remaining 4 wings. They can be bought with money too but being able to unlock with in-game gold is really nice. I think Hearthstone has emulated the free-to-play system of League of Legends really well; you don't need to spend money in the game to get the full experience.
Update: It's out! You need to unlock all 9 heroes to access the expansion.
