It seems like every movie that isn't a straight documentary or romantic comedy is required to be made in 3-D lately (although the coming Harry Potter will likely not be), and there's even talk of re-releasing the Star Wars series in 3-D.
I haven't been able to find a general comparison between 3-D and 2-D ticket sales, but I've heard a lot of people including Roger Ebert, say they don't like watching most movies in 3-D. But why?
Well, for one thing, it usually gets very gimmicky, with random stuff flying out at you, which gets annoying, and the scene gets crowded and chaotic very quickly. Generally, standing in front of people going "look at me, look what I can do, look, look, look!" gets old pretty quick.
Also, picture quality tends to suffer quite a bit, with a loss in brightness and muddy special effects (one of the major critical complaints about The Last Airbender was that it was visually murky and hard to follow - when seen in 3-D, I saw it in 2-D and it wasn't nearly as bad as I'd read). This can be compounded by the one-size-fits-all 3-D glasses used by theaters. If they don't fit, you'll get a blurry picture, headache, and possibly motion sickness (they don't fit me very well, they're too close together so I'm just about cross-eyed the whole time).
And about those glasses... they're goofy looking and they act as a barrier between you and the screen, as well as your fellow movie-goers. Part of the fun of a good movie is being in an audience and sharing in a collective gasp or giggle, adding a layer of uncomfortable plastic puts up a psychological barrier to that (and, speaking of glasses, what do people who normally need to wear prescription glasses do?).
As Ebert points out, part of the art of traditional movie-making is manipulating the image to make us believe in the two-dimensional projection. Our minds fill in the gaps, making for a very convincing illusion. 3-D is not really unnecessary and tends to look LESS believable in a lot of circumstances.
While 3-D can work well for certain films in certain circumstances, in most cases it seems like nothing other than an excuse to jack up ticket prices. Here's hoping that "all 3-D, all the time" is NOT the wave of the future, I shudder to imagine watching Casablanca and having Rick's cigarette zooming out at me 10 feet long. And the Nazis. Please, please PLEASE can we not have 3-D Nazis?