There just isn't enough time for every character to be well defined. Magneto's henchmen, Mystique, the lion-like Sabretooth and the aptly named Toad receive even less development. Mystique is almost exclusively a special effect, albeit a fascinating one. Naked and blue skinned she is both sultry and deadly. Sabretooth's powers are poorly defined and mostly appears to be a thug.
Toad is fun to watch.
I brought along a friend for the movie who was almost completely unfamiliar with the comic book. He came away impressed and saying it was pretty damn cool. I mention this because the acid test of such a movie is how it works for people not already familiar with the characters and story. X-Men stands on it's own merits as a movie. No previous experience with the comic is necessary to enjoy it.