Some people are complaining that some 3D movies are darker than other 3D movies. I dont know. I never noticed anything. Could be this true or just somebodys feeling? Is this caused by particular 3D technology or what?
I also read some post which claimed that some converted 3D movies are faking that "3D feeling" by blurring background? I dont think thats true, but maybe I'm wrong?

Of course there is a luminance loss (at the least) through the eyewear. Even the passive glasses will filter out some light, actives, that only allow the light energy to one eye at the time loose even more of the light you perceive. Anaglyphs have even more artifacts, and light loss is not the biggest issue, there is significant chroma loss. Theaters and TVs often compensate for the loss of the luma, but that leaves some artifacts due to lesser dynamic range of the remaining light. In TVs there is lately even some concern about the additional energy requirement for this increased luminance needed for 3D.

Stereoscopic 3D doesn't need to blur the background to get the 3D impression, but it may help in guiding the audience to concentrate on the portion of the scene the director wants them to concentrate on. It may also help if the background parallax gets to be too wide for very large screens.

Yes, I know that 3D movies could be darker because of glasses, but could be one particular 3D movie (for example Clash of the Titans) be darker than other 3D movie, or its just caused by particular 3D glasses? I didnt noticed any big difference by watching 3D movies, but some people claimed that (for example) Clash of the Titans in 3D seems to be darker than other movies in 3D. I thought its a nonsense and it could be probably caused by particular glasses or maybe particular screen which is not adequately reflective...? but maybe they are right?