The movie was a good idea and unique narrative/visual construction, but it failed a crucial test of all potentially great films...namely, that the audience never identifies or develops a rapport with the main character (Leo's).
In INCEPTION, the goal to get back to the kids isn't fully fleshed out until the third act...and by then it is far too late.
When one realizes this is actually just a bad heist genre film, plot/character-wise, you'll understand why so many people fall asleep at the end of the first act.
A film that demands this much mental energy MUST connect to the audience, so they will go along with the ride willingly. If you were watching INCEPTION on your TV for the first time late at night, you'd most likely fall asleep or turn it off within the first 1/2 hour.
Contrast this to THE MATRIX (the first, classic one, not the knock off sequel crapolas). We learn very quickly that the fate of the whole human race is at stake...and almost more importantly, the meaning of life/existence is hinted at right from the outset. We all identify with those questions, etc. so we're hooked on wanting to know more BEFORE the big twist is dropped on us.
Simply put, INCEPTION is a very visually stylistic OCEANS 11, but without the strong character connection of even that lessor effort.
In 10 years, you will be still be watching THE MATRIX from time to time. Is there really any reason, even now, to watch INCEPTION more than once? Think about it.
That's the difference between truly great movies and movies that only pretend to be great.