With books, I tend to impulsively avoid anything with cover art that looks like it's trying too hard to be "cool", "dark", or "badass", because I assume that they'll be pandering schlock that claim to be targeted at "young adults", but are really written for someone of the mental age of 13. I've heard such great things about the Night Angel trilogy, a lot of it from other Escapists, but seeing the cover always activates my bias, and I haven't been able to bring myself to buy it yet.
As for people, if someone says something along the lines of "reading sucks", I tend to think far less of their intelligence. I know that isn't really fair, and I'm trying to work on fixing that.
I feel really bad about saying this one, but I tend to assume that obese children will be incredibly obnoxious. I really don't know why I have this bias, but I do. Thankfully, I've never acted on it, but being so reflexively annoyed by them does make me wonder, "what the fuck is wrong with me?". Again, I'm trying to fix this one.