1. All you do is repost ("retweet") other people's shit.
Hey, guess what? If I wanted to follow all those other people, I fucking would. I follow you because, for some reason, I was convinced that you had original thoughts to share that I would be interested in. When the majority of your tweets were written by someone else, I question your ability to think for yourself and, further, I question why I'm following you in the first place.
2. You reply to others by reposting their tweet with your response attached.
Every now & again, especially if it's a conversation you think might be informative to others, that's no big deal. Carrying on a multi-tweet conversation this way is really fucking annoying to everyone else that follows you. In case you were unaware, there's a magic new button on twitter now that allows you to "follow conversation". You can actually see the entire thing in one place without making it ridiculously painful to follow.
3. You don't interact with others.
The best part of twitter is the part where we talk to each other. I've said before that twitter is like a chat room where we get to pick the participants. So participate! I'm not saying you have to respond to every fucking tweet, but if someome replies to you directly, you should, you know. Respond. Acknowledge that person. It's polite, asshole.
4. Your twitter and facebook accounts feed each other content.
I can see this being useful some of the time. But if you set this up so you can ignore one and focus on the other, that's just stupid. If I follow you on both and they're always the same thing, I will pick one. If your twitter always runs too long and I have to continue to facebook to read the rest? Yes, I'm dropping your twitter account. Please understand these are two different and unique platforms. Use each for its strengths or pick the one that best suits the way you plan to interact.
5. All you do is whine
We all have our bad, sad, cranky, crabby days. If people unfollowed me for mine, I wouldn't have followers. But nobody wants to follow someone who is consistently whiney, grouchy, snively. Debbie Downer has no friends for this reason. If you genuinely are this unhappy with your life, you need to change some things. Bitching about it isn't helping.
6. Your tweets are all trying to sell me something
Word to the wise: even the businesses I follow on twitter don't do this. And that's why I follow them. I'm not on twitter for commercials and I don't follow spambots. If you're a spambot disguised as a regular guy who likes similar things to me, you're just pissing me off.
7. I've ever read one of your tweets and thought "shut the fuck up"
This is totally subjective, but I'm sure you understand exactly what I mean.
Look, I don't care if you're drinking the Pinterest Kool Aid, but I don't need that shit cluttering up my feed. Keep it off facebook, too. It has its own dedicated site for a fucking reason.
What are some of your reasons for unfollowing someone on twitter?