I love all things steampunk, so I borrowed Steamboy from the public library recently.
Story: The plot had all the elements of a good plot: multiple characters whose actions collided in an exciting and suspenseful way, excellent pacing wherein no scene felt unnecessary or unnecessarily long, and the situation kept getting worse and worse until the climax, which was, admittedly, rather vague. Furthermore, the plot managed to incorporate complex themes, framed through the characters and their struggles with each other because of their beliefs about science and society. And managed to do so without long philosophical lectures.
Animation: Steamboy is one of the most expensive anime films ever produced, and it really shows in the art. The backgrounds were unbelievably gorgeous, textured, detailed, dimensional, and had a great sense of depth and perspective. Gears and valves and steam were plentiful. The explosions and things being smashed and broken looked cool. The character design was a little plain, but worked.
Sound: Nothing overly groundbreaking. The soundtrack fit comfortably, and the steam and gear noises were fun.
Characters: Although most of them fit one trope or another, I liked that their personalities were revealed through the actions they took and through their conversations with one another--instead of through narration or monologue or flashback (which, in my opinion, are much less effective ways to develop characters).
[Inner Feminist objected to the lack of female scientists, and the lack of sensible females in general. Sure, you can argue that it makes the film's atmosphere more "historically accurate," (because obviously women never were nor are sensible) but the fact of the matter is that we are in 2012 and rehashing stereotypes in the name of "historical accuracy"--especially considering the license taken with the bounds of scientific reality in this film--gets us nowhere.]
Overall: Great Gears of Steam and Fire! This film is completely worth 2 hours of your time.