Thor is a 3D conversion, not an original 3D production.
This is confirmed by multiple sources:
- http://www.youtube.com/watch?v=U… (first question, answered by director Kenneth Branagh, who argues that conversion is less “headachey” than stereoscopic 3D)
- http://screenrant.com/thor-capta…
- http://blog.moviefone.com/2010/0…
- http://www.the3dmovies.info/2011…
The quality level is said to be vastly superior to Clash of the Titans – but since none of the movie was shot with 3D in mind, the special glasses you wear will do more to take you out of the experience than make you feel like a part of it (“hey look, a FLYING HAMMER that goes… uh… SIDEWAYS, in 3D!”).
If you want to be cynical, you could say that it’s basically a money grab, and the reviews from those who have seen both say that the resolution and colors/blacks onscreen are truer and more vibrant in 2D… But if you enjoy the 3D experience generally, and don’t need excessive screen-popping action, it’s a good way to ensure a less crowded theater when you go to see it.