I think the issue I have with Tom Hanks is that he strikes me as a competent actor. I can't think of a single role I've seen him perform that he has taken over and really made his own. Actually, let me take that back: I can't think of a single dramatic role he's done that stands out as a signature performance. The character he played in Big had more depth and individuality than the character he played in Saving Private Ryan. Hell, he was more convincing in You've Got Mail. I'll admit that I might be guilty of type-casting the man, but he has yet to convince me as a dramatic actor. Contrast this with Robin Williams, who should only act in subdued, dramatic roles, or Helen Hunt, who single-handedly turned As Good As It Gets into a touching film.
Tom Hanks isn't overrated; he's perpetually miscast. I shudder to think about what's going to happen to his career if more people discover this.