EDIT: I now see how we may get other heroes to play as, such as Illidan who was shown in the trailer...
Practice mode: Hone your abilities and test out new deck ideas against A.I. opponents of basic or expert skill levels, and unlock new playable heroes in the process.
Seems like you can unlock new heroes simply by practicing. Or I guess it could mean you can't practice with all current heroes right off the bat?
Note the bottom where it says "standard decks". I think it's fair to assume that hitting the arrow would take us to "intermediate" decks, or a similar name. I'm wondering if this is where other heroes such as Illidan make an appearance. It seemed odd for me to see him so clearly in the trailer but not as a selectable character. All I can think is that he might be the avatar of a more complex Warlock hero or something along those lines. What do you think? A different hero avatar for the more complex the deck? Or am I thinking too much and the next step across is just decks made by the player? lol Curiosity... it kills me :P