There's a human tendency to resent anyone who disagrees with our pleasures. The less mature interpret that as a personal attack on themselves. They're looking for support and vindication.
No movie (or anything or anyone else) is liked by everyone. That is simply the nature of the beast (or the reality of having too many humans living on earth).
I am glad that I did not read any of the massive hype about "Inception" before I saw it, so that I had the chance to take it on its face value and preserve my first, uninfluenced impression.
People are very susceptible to suggestion and hint from their environment. I know I am very susceptible. In that sense, the act of "inception," as Nolan defines it (ie, planting an idea in someone's head to make him believe it is his own), is very very common and need no fancy gadgets and men with big guns to achieve. It happens every day.
"Inception" has many flaws, that is for sure. There are many elements for which I can criticize it. And it is true that Nolan's morality (or lack of) can be a bit disturbing to some, me included. Nevertheless, like Roger Ebert, I do not believe in perfection. I do not believe in flawlessness. A film that is not disliked by someone out there is not worth making or watching.