- A stylistic avatar. The best feeds have an avatar (little profile picture) that someone has put some thought into and that reflect, in some simple way, the content of the feed. Less interesting feeds just have a bland picture of the author. To a certain extent you can judge a book by it's cover! Just by scanning through avatars, you can get an idea of what feeds might be worth following from how interesting the avatar is.
- Talk about universal truths rather than what you are doing. Do I care that you are currently at Starbucks on Main Street? Give me something I can use! Unless you intend your feed for only your five closest joined-at-the-hip buddies, no one really cares what you are doing.
- Personally, I believe your "what I'm doing" stuff should go on Facebook. Even then, you should limit your updates to things that are somehow useful to others.
- Limit the use of "I" and "me". Again, what you are doing may be interesting to you, but the rest of us want information we can use in our own life. "I" and "me" should be used only sparingly in certain rhetorical situations or to distinguish your work from someone else's.
- All links should be explained. Don't just tell me to "Look at this!" You have to give me a reason to look (because clicking on each link takes time, especially on a mobile device).
- No wasted words. There should be not a single unnecessary word in any tweet. Even if you have extra space available in your 140-character allotment, the tweet should be as compact and efficient as possible. Before sending the tweet, read and re-read it to make sure you have cut it to the bone. Extra words just dilute your message.
- Interesting language. Don't say something in a bland way if you can say it in an interesting and stylized way, with clever and precisely targeted language.
- Only one idea per tweet. Don't try to insert multiple ideas into the same tweet. Know when to stop!
- Every tweet should stand alone. Although two adjacent tweets can be on a related topic, they shouldn't depend on each other for their meaning.
- Every tweet should be timeless. The best tweets should be just as meaningful when read ten years from now as they are today. If the tweet must comment on a current event, it should contain enough information about that event to allow the future reader to decode it (with some help from Google).
- Avoid hash tags. I think they just clutter up the tweet and make it less timeless. I admit I don't have much experience with them, but I doubt they help anyone find your tweet. (What sense is there in commenting on #obama if a million people are using that hashtag too?) I suspect that most readers rarely do searches by hashtag but merely read the feeds they are subscribed to; in that case the hashtag is just noise.
- Avoid abbreviations. I you have to use abbreviations to fit your tweet into 140 characters, your tweet is probably too long anyway. God gave us only 140 characters for a reason, and you shouldn't try to fudge His limit.
- Use proper punctuation and capitalization. It makes you look like you care.
- Low sexual content. Sex is the world's most overused topic. Tweets that consistently dwell on sex are a sign of weak imagination and are ultimately boring. Let it go!
- No moping! Even if you're funny, your readers don't need to know how badly your life is going. If you have a problem, solve it. Otherwise, don't expect affirmation from us for your drinking, career problems or lack of sex lately. Honestly, tweeting something funny about is just an evasion mechanism. Do something!
- Don't feel the need to be funny. Comedy can be a crutch and a prison, especially if your followers expect you to be funny in every tweet. It is more important to be observant and useful. Don't set up expectations for entertainment that you can't reasonably fulfill.
- Know your focus! Every feed should have a central topic. This is your central core - your Twitter personality - that you are always returning to. Over time, people should be able to distinguish your tweets from everyone else's from both their content and their style. There are plenty of "general" Twitterers out there, commenting on the news and celebrity scandals of the day. Your feed should have a unique identity that doesn't depend on these topics.
Here is some information on my own Twitter feed including my tweet archives. My best tweets respect the rules above.