I will follow anyone who follows me, with the exception of:
1. MLMers
2. Mommy bloggers who only tweet about baby products all day
3. Anti-gay folks (of course!)
4. Someone who only posts links to their blog, but never actually converse with other people
5. Someone who follows me, waits for me to follow back, then unfollows me (I knowwhat you're doin'!)
Sometimes I do a search to find people who are tweetin' about things like DOMA and Prop 8. I follow those people because we have similar interests.
I will only follow them if they have a pic, a real name and the link on their profile doesn't direct me to some lame "RECESSION PROOF JOB/MAKE MILLIONS" type of thing.
I tend to follow people who have [appropriate] avatars, filled in their profile, don't have spammy links. Usually I'll follow them if they send an @thaoly my way so then I can converse or follow them back. I think auto-following just fills my stream w/ too much noise.