According to my sister, I scowl and can look pretty intimidating when I'm concentrating. (heavy eyebrows and somewhat deep-set eyes'll do that) When she gets worried is when my face goes blank. That means I'm MAD.
Oh yeah- I forgot about the security guard at work asking me why I always looked like I wanted to punch someone. I figured the best answer wasn't "Because I do" and I worked on moderating my expression a bit more after that.