I had a deck based on mercenaries, Attrition, and Avatar of woe. It was fantastic, for a deck that was almost completely MBC. Avatar of woe is a very useful card, however it is not powerful enough to define a format. In order to be amazing a card must have 3 qualities:
1: Immediate impact.
Get it in play, and the game changes. It's a target all of a sudden. It must be able to come out on demand, usually the lower CC creatures do this better. Blastoderm, for example. Avatar has problems with this. Many times you devote much of the deck to solving the CC on this creature. Not a good thing. Exceptions include Verdant force, via Natural Order.
2: Lasting impact.
Leaves it's mark on the game, even if it dies. Avatar of woe does this VERY well. There are better cards, but not many. Verdant force is high on the list. Morphling scores low in this one area.
3: Utility.
I don't mean minor utility, I mean serious utility. Avatar of Woe is mediocre in this regard. When I say utility I mean Morphling-style utility is high, Verdant force/tradewind rider is good. Avatar is useful, just a little slow. And it has only one inherent defense: being black.
Notice the Verdant Force (the best fattie ever printed) is on all three. It just happened like that.