I find that I often play stern brooding commanders in regards to personality; almost always human in race. Different races never really appealed to me nor did super natural powers. I think self typecasting is normally just the result of a creator's desire to role play a character which they themselves are not only comfortable with but also enjoying playing and a person will usually be drawn to create a character using similar, if not the same personality traits and niches because of this.

Being typecast by others is a whole another subject.