Mashable has a great post, FOLLOW FAIL: The Top 10 Reasons I Will Not Follow You in Return on Twitter, with reasons that are valid for me too. Each reason is expanded in their post, the list goes like this:
1. You have no user avatar
2. You list no location, no website, or no bio
3. Your “website” listed is a MySpace profile
4. You’re following over 1,000 users, have 20 followers, and no updates
5. Your profile features any variation of “Internet expert”
6. Your updates clearly indicate that your Twitter activity is always, only, about pushing your own service/product
7. Your following and my return follow result in a poorly-constructed auto-DM reading, “Thx for the follow! How can I help you get to a 4-Hour Work Week?”
8. Your most recent updates make references to any need to achieve “more Twitter followers”
9. Your Twitter stream indicates a propensity for consistent arguing
10. You do not engage your Twitter followers

Number 6 is annoying and a reason for me to un-follow.