This is more of a general Marvel Cinematic Universe post than anything strictly related to Age of Ultron, but I'll start with an easy one. We've seen four of the Infinity Stones in the movie, we know the one inside Loki's scepter (and now sitting on the Vision's forehead) is the Mind Stone.
So which ones do the other three correspond to? I figured the one inside the Cosmic Cube is the Space Gem, since it opens doorways to other places. The purple one from Guardians of the Galaxy converts the energy in any living thing it touches into destructive force, so, Power Gem? Which leaves whatever the red gem in Thor: The Dark World was. Doesn't seem like it would be Soul, and certainly not time, which leaves Reality? It's hard to say, rewatching the movie recently, it seemed as though Malekith mostly just used its power to throw swarms of rocks at Thor, which is kind of vague.