Personally, I found Vol. 2 to be a far superior movie to the first. I found the first one to be infuriatingly shallow. It had a plot and characters that were given about 5 minutes of thought, a flaw which QT attempted to fix with high production values and 45 minute action scenes. To me it represented everything that I find wrong with so many Hollywood movies these days - the perfect example of style over substance. As far as depth goes, it was on par with a Vin Deasel movie.
The second one took an entirely different approach. Characterization was top, action was just a means to an end. Exactly as it should be, in my opinion. Although sometimes it dipped back into the super-cheese (the Pai Mei scenes come to mind - say what you will, but if I wanted to watch old 70's cinematography techniques and characters, I'd go rent a 70's kung-fu movie), it generally stayed, as somebody else previously mentioned, much more grounded. The relationship between Bill and "The Bride" was very interesting to watch. She loves and hates the man at the exact same time, which leads to some very powerful moments. Don't get me wrong, the movie had it's flaws. Some of the pacing was a bit slow here and there (particularly during the beginning) and I didn't care for some of QT's "experimental" techniques (mostly because he wasn't really experimenting, merely "borrowing" them from more risky film makers) but it was a huge step up from the first one. No, it isn't one of those "hang your brain up at the door and enjoy" types of films that you can watch over and over again on DVD (as the first one was supposed to be), but that's exactly what I liked most about it.
Worst part about this movie: the revealing of The Bride's name. So much build up for a very weak payoff.
-Matt