Using Ryan's criteria, which I understand it was 5 released, exclusive, "must-have" Vita games:
Gravity Rush
LittleBigPlanet
Lumines: Electronic Symphony
And I'm done.
Sony is leaning heavily on the portability factor, while Ryan's emphasis was on games. If you take out the exclusive requirement you get really good portable versions of Rayman Origins, Mortal Kombat, BlazBlue, Tales From Space: Mutant Blobs Attack, Sound Shapes, Ultimate Marvel vs. Capcom 3 and Disgaea. If portability is a high priority to you, Vita isn't quite as barren.