Inception is a bad movie. I'm going to skip over some complaints that aren't a big deal, then I'll address the main problem.
Dreams are completely different in real life and in Inception in basically every way. Someone pointed out that the movie would work much better if they said "subconscious" instead of "dreams", so let's just pretend that's what they did.
The rules throughout the movie are completely inconsistent, and often ludicrous. Falling in your sleeping body is supposed to pull you out of your dream, but later in the movie, they pull a switcheroo and have the dream person fall to wake up the sleeping body. The entire Limbo thing was particularly dumb, if you can just kill yourself in Limbo to wake up, then Limbo isn't a real threat, or it wouldn't be if they were being consistent. The increasing speed at lower levels is an interesting idea, but completely unjustified by the (incorrect) idea that the brain can work hundreds of time faster than it normally does*.
The naming of the characters is particularly silly. Ariadne? Are you serious?
In defense of Inception, it's not actually that confusing or hard to keep track of or understand: Some guys use a magic dream penetrating thingy to insert the idea of breaking up a company into a businessman's psyche.
The idea that there's so much "meaning" and "interpretation" to be analyzed is just piffle. There's no subtext whatsoever, and the only interpretation anyone makes is "was scene X a dream or reality?" A popular interpretation is that the entire film was a dream, which could be said of any movie whatsoever, and is therefore completely inane. There's no logic to think that the "real world" of the movie is real or dream, there's no system to determine what's real or not. The most popular dream question is the final scene, which must be a dream, unless Christopher Nolan would have us forget that children age in the real world. The time that the apartment across the street was the same apartment that Dicaprio was in was an interesting, almost subliminal, visual trick, but it doesn't mean anything.
Now the big stuff, starting with the second largest problem: The "antagonists". Inception is interesting in that it has no character serving the rolê of antagonist, the obstacles they have to work against are psychological. At some point, the people making the movie thought this would be boring, so someone decided that dreams defend themselves with gun wielding characters who attempt to eject foreign dreamers. This makes for a constant video-game-esque gunfight against faceless goons for absolutely no reason. This provides no real narrative pressure or threat, since it's a dream, and we don't care about the protagonists.
That's the largest problem with Inception, we don't care about the protagonists**. There are only two people in the movie who actually have character, and they're both rather obnoxious. The businessman is just a stuck up clichè. Leonardo Dicaprio's character, oddly named Dom Cobb(?), is the person that we're supposed to sympathize with, but he's totally uninteresting. Without an antagonist, and without a sympathetic protagonist, and with action that's almost entirely in someone's subconscious, we're left with a plot about no one in a conflict with no one over nothing in a place that isn't real. No wonder I didn't care about what happened. For such a long movie, I'd like more payoff than "unsympathetic Dom Cobb gets over the guilt of his wife's death".***
I've heard people say that they were hit hard when Cobb actually got to see his children's faces. Maybe I'm just cynical in my old age, but they didn't see that coming from a million miles away? It's so shamelessly manipulative and predictable, I can't believe people accepted that as resolution.
PS. I don't care whether the end was in a dream or not, but the way the top wobbled just a little, then ending with a cut to black, that was genius. It's very rare you hear everyone in a theater groan. Even I had a visceral reaction to that trick.
* We do not use only 10% of our brains, a myth the movie alludes to without actually quoting. There's no way we would evolve the ability to think hundreds of times faster without the potential to do it. Evolution doesn't work that way. Interesting side note: they should hook up geniuses to the dream stuff to have them do centuries of theoretical mathematics over the course of a few months.
** We also don't care about the mission. "Stop an enormous energy company from getting larger!" That's our rallying cry? Really?
*** Why didn't Dom just tell his wife that he had Incepted the idea that they were in a dream? "Honey, I stuck that idea in your head, that's why you can't get over the idea," is better than your wife killing herself.**** And why are there so many rules about Inception if no one else had ever gotten it to work before?
****Maybe we're just not supposed to notice, but every time it cuts to Dom and his wife waking up from their Limbo dream, they don't have any of the dream sharing equipment.