No profile photo... of a human being. If you don't have a photo of you (not your cat or dog or fish or logo) I probably won't follow you. Twitter is about developing relationships with people, not logos or cats.
Your Twitter ID is a company name. Again, Twitter is about relationships with people, not companies. If you have a company name, that account should only be used to broadcast company updates or have very company specific discussions. My personal preference on Twitter is to have relationships with people.
All broadcast, no conversation. If you have a stream of updates on Twitter that does not include any @ replies, then I probably won't follow you. You're just not interesting, you're using Twitter just to broadcast your messages without interaction.
Follow-Me-Back-Nazis. These are the people that follow thousands of people hoping that some will follow them back. these are the same people that get mad at you and sometimes send you harassing messages if you don't follow them back. Look, just because you follow me does not mean I want to follow you. In fact, I only follow back about half the people that follow me. I look at your profile, check your recent tweets and bio, and decide if what you are saying is useful to me. Not useful, no follow. Sorry. Stop harassing me! You can unfollow me if you like.
TMI (aka Too Much Information). Look, I like how Twitter is a combination of some business and some personal info so I can get to know people on both levels. But there are limits. I do not care if the burrito you ate for lunch is too spicy. I do not care if your cat is feeling sad. I do not care if your computer is slow. I do care where you are hanging out, if you are attending a cool event, if there is a major development in your life (wedding, baby, moving, job) and if you find an article online that is particularly interesting. Try to think about if your tweets are interesting... at all.
Anything I missed that annoys you on Twitter? Leave a comment below and let's continue the list!