But it's the flip-side that really makes this new Avacyn interesting. Whenever a non-Angel creature you control dies, you transform Avacyn at the beginning of the next upkeep. This means that you could Flash her in during an opponent's turn after one of your creatures die. She'll then transform as your turn begins. Avacyn becomes the Purifier, who deals 3 damage to each other creature and each opponent as she transforms.
The effect seems counter-intuitive since your creatures will no longer have indestructible at that point. There is Make a Stand from Oath of the Gatewatch, an 2W Instant that makes creatures you control gain +1/+0 and indestructible until end of turn. Also, Avacyn becomes a 6/5, although loses vigilance.
This new Avacyn is like a super-charged Serra Angel. She seems playable in a Red/White/X mid-range deck, although you don't actually need to play Red to play her in Constructed. Red/White/X Commander decks will really have fun with this new Avacyn, though, especially Kaalia of the Vast. She could prove to be a fairly good Commander herself, especially with Flash. Timing her ability is very important, but she can be pretty devastating.
In most cases, Avacyn the Purifier will wipe the board, deal 3 to your opponent, then swing for 6 more. Those 9 points of damage will likely be enough to win the game most times. Some players have noted that it doesn't seem fair that she doesn't deal the 3 damage to you - flavor wise. But this was probably done for play-ability reasons. She strikes me as a land stand sort of card that can turn things around when you're behind. Saving you from a board wipe seems worth it. You just have to be careful how and when she transforms. It's possible she'll be a format-defining card. Heck, she may even be playable in Modern. Only time will tell.