It’s the waste of a dimension.
It adds nothing to the experience.
It can be a distraction.
It can create nausea and headaches.
Have you noticed that 3-D seems a little dim?
There’s money to be made in selling new digital projectors.
Theaters slap on a surcharge of $5 to $7.50 for 3-D.
I cannot imagine a serious drama, such as Up in the Air or The Hurt Locker, in 3-D.
Whenever Hollywood has felt threatened, it has turned to technology: sound, color, widescreen, Cinerama, 3-D, stereophonic sound, and now 3-D again.

The only 3D movie that I’ve seen is Avatar and I really enjoyed the experience. But I also have read that James Cameron did a superb job in integrating 3D into Avatar, avoiding a lot of the reasons why Ebert hates 3D.