Most common misconception IRL seems to be that since I'm usually excited, animated, and eager it must mean I'm immature, naive, or completely stupid. I just think life is what you make it, and there is awesomeness to be found in just about anything if you're open to it.
On forums, I'm often mistaken as a troll for a lot of the same reasons.
I'm bad at pegging anyone's conception of me on the whole though. I try not to assume unless someone outright tells me something, and people rarely tell you what they really think of you.