I think Akira's a touch long, more than anything-- I can accept "WEIRD SHIT HAPPENS!" as a movie premise, as long as you concentrate on the weird shit and a couple of strong characterizations. It's just when they keep injecting rather incomplete and subsequently pointless background information into the proceedings that the movie really loses my attention. As I said before, I think a firmer emphasis on the friendship/rivalry between the oblivious Kaneda and the passive-aggressive Tetsuo followed by jumping straight to balls-to-the-wall weirdness and climactic ending with some of the more pointless exposition removed would have made for a tighter movie, and I don't think it would really have lost much because the movie is remembered for it's excellent animation and art direction more than anything.
It's understating things a bit to say that it was American audiences who were unused to such visuals-- Akira was a highly polished effort by any standard, with over a 150,000 cells of animation and it was one of the few Japanese projects (maybe even the first? I forget) in which voice actors were recorded first with the animation produced to fit the voices instead of vice versa-- it may be standard procedure in many American productions and Disney in particular, but in Japan the prohibitive cost made the stereotypical poor dubbing one of the many cost cutting maneuvers that contributed to an overall lower standard of animation. That's not to denigrate the artists, mind you, but the fact of the matter is that most animation studios simply didn't have the budget for the kind of polished effort you'd see from a Disney or Don Bluth project of the same era, so Akira really was something of a coming out party.