I’m a girl, and I’ve always played female characters. I like playing female characters because they bring a different dynamic to a group of male characters and because I don’t feel...qualified, I guess, to play a male character. Yet, I’ve been in several games where women have chosen to play male characters, and men have chosen to play female characters. At the worst, gender has been either ignored or treated like a stat; rarely have I seen someone convincingly play a character with a gender different to their own.
Of course, I don’t mean to say that it can’t be done, nor do I mean to say that it shouldn’t be done. The motivation behind playing a transgender character is, however, a mystery to me. I should like to be enlightened! :)