3-D isn't quite as prominent in movies these days, but from about 2009 to 2012, it was everywhere.
Pretty much every studio release, from big-budget live-action blockbuster to an animated kids film, was released in 3-D, with varying results. Even 3-D Blu-rays were a thing for a little bit. Does anyone remember those? Do they still release 3-D Blu-rays?
Anyway, Inception was released at the height of the craze in the summer of 2010. Given the big-budget nature of the film and its incredible action sequences and visual effects, you'd think it would have been released in 3-D, especially since it was at the craze's peak.
However, according to cinematographer Wally Pfister, Christopher Nolan refused to film the movie in 3-D. Nolan said that it would distract from the storytelling experience of the film, a sentiment I think plenty of us will agree with.