Maybe "perfection" is too much praise. There are a few flaws: Lotso Bear's good guy / bad guy duality and secret past come off a little blandly considering the growing depth of the other characters. CGI in all computer-animated cartoons, not just Pixar's, can't quite seem to get a decent looking human being on screen yet. Of course, Pixar has always turned this lack into a strength -- what better way to identify with a bunch of toys than make them the most expressive characters in the movie? And, as I said at the beginning, it leans heavily on the character-building work done in the previous two movies. But that's okay, too. No one's going to start with this one, anyways.
Maybe I'm just tempering my own love of the movie by wracking my brain for flaws to pick at. Honestly, I'm surprised by how much it effected me. It's been, what, over a week since I've seen the film and still, by total surprise, the fear and love and hopelessness and whatever other emotions that rushed over me in the final ten or so minutes of the film, all of these things still just pop up every once in a while in the middle of my day. And I think that's the mark of a really impressive movie. A great movie, in fact.
Pixar is in a unique position. They are, as near as I can tell, an almost independent studio backed by Disney-sized stacks of money. It is a very rare arrangement, and one that I am unsure will ever happen again. My suggestion to everyone watching is to enjoy the ride while it lasts. It won't be here forever, but you can still love it for the rest of your life.