Battery life, speaker placement (my hands cover the speakers), the L&R buttons (which I don't personally like), and weight (it's not heavy, but it could probably be lighter) are the only flaws I can think of.
I owned a PSP 1000 and a 3000, and the difference was like night and day. The 1000's screen was dull and dark, whereas the 3000's screen was much brighter and more colorful, and the Vita's screen is the evolution of that. Even on the lowest brightness setting, it's really bright, and if I could make a complete wild comparison, the lowest brightness setting looks the highest brightness setting on my 3DS.
I don't know how likely a Vita revision is, but unlike with other consoles, I don't think that the Vita's disappointing sales are due to hardware. The hardware is the best part of the Vita, and the reason people keep shouting for more games and port begging is because it feels so great to use, and I want to use my Vita more.