I do think his nerdiness is part of it, but more than that, he's just a normal human guy that we can relate to. His problems are real but not earth-shattering. So for example, his boss may be a jerk, but he's not going to come home and find his wife in the refrigerator.
Another thing is that his powers are cool, but not cosmic level so we can fantasize about having them, but still be able to imagine what it would be like.