dtgreene: From what I have played, my one complaint is that the levels are too long, and there are no mid-level saves. As a result, this game isn't suitable for when I just have a few minutes to play.
I guess that one is a matter of expectations. If you went in expecting short levels I can see your point, but in the grant scheme of things we're talking less than half an hour per level, which is fairly on par with a typical game.
(Actually, I could give another complaint: there's too much of a difficulty jump between the non-boss portion of a stage and the boss fight.)
Totally agree with this one. The levels are all fairly subdued, and you'll rarely (if ever) actually lose a life while playing through them. Those bosses, though? Going through 5-10 lives per boss is fairly normal.
The ridiculous difficulty of the boss fights made the speed-running achievements feel empty. It was all about beating the boss on the first try, and the rest of the level just felt like irrelevant filler. You just don't care about shaving off 10 seconds by recklessly dashing through a difficult section when a screw-up on the boss battle will cost you to 2-3 minutes. That boss battle becomes all that matters.