I haven't ever really decided not to like someone solely based on their username. Well, okay, there were a couple that gave me the impression that the persons behind them was either quite young, or quite uneducated, but the posts made by said persons are really what proved me correct.

On the subject of gender, though, I also thought TheAbominatrix was male when I first joined. Her avatar at the time was a soldier, and the same soldier was in her sig, I believe. I think there was a post she made once that really made me think of her as male, though; I can't really remember.

I also thought Farah was a female. I actually only found out a couple of weeks ago that he's not. xD

I also for some reason thought proto was tall
Me too, not sure why.