Here's a tip for movie studios:
If you are aren't planning on 3D effects for the entire film, or if you are thinking about adding 3D post-production, just say no.
It becomes distracting to the viewers if the film switches back and forth from 3D to normal projection because some scenes are just too difficult or expensive to transfer to 3D.
Case in point: I loved The Avengers. There is literally nothing I could say about the film that hasn't already been said thousands of times already by people a helluva lot smarter than me (they wish). My one quibble with it was the post-production 3D*. It made no sense. For example, there was a car chase near the beginning of the film. The chase itself was not in 3D, but every time the perspective switched to inside one of the vehicles the 3D kicked in.
What the fucking fuck is that all about? It was inconsistent at best and distracting at worst.
Just say no. Seriously.
By the way, Dave2 had a great point (spoiler alert) about another minor quibble with the film. I found it even more minor than he did, and I actually had a different issue with the same point. But, like I said, it was minor. The Avengers was a nearly flawless movie-going experience. Notice I didn't say that it was a nearly flawless film. Hard to judge that yet, but as a movie-going experience it ranks up there with Terminator 2: Judgement Day, LotR: The Fellowship of the Ring and Avatar. Three movies that made me shake my head and mouth "holy crap!" over and over and over again while watching them in the theater. Doesn't mean they were all incredible films, although I think the first two are. Avatar, besides the incredible effects, was largely forgettable for me. But that didn't distract from how awesome it was to see it in the theater.
The Avengers was like that. Probably the best super-hero movie I've ever seen. We'll reserve judgement on that until it has had some time to sink in. Just go see it!
*I was warned the night before by several folks to skip the 3D version of the film, but I had already purchased my tix for the next day through Fandango. You live, you learn.