Found this an important short list, excerpt from Futurist Ross Dawson's post on March 31st, 2012:
WHAT TO TWEET
* Interesting, current links with useful descriptions. The most valuable tweets usually point to fantastic content, with sufficient description for people to know why they should click on it.
* Links to your own content. People like links to content created by the Twitter account owner, which is why they follow them.
* Stimulating questions. People find value in interesting and provocative questions, whether or not they respond on Twitter.
* Occasional unusual or humourous posts. It is good to break up the flow of a Twitter account with funny or different posts, rather than have it be too consistent.
* Responses to others. Twitter is a conversation, so it is important to respond to others, and these can be among the most interesting and informative posts.
WHAT NOT TO TWEET
* Content-free statements. Don’t say things like hello and goodbye, or other interjections that contain no content and add no value.
* Excessive personal updates. Unless your Twitter circle is only close friends, don’t just tweet your day-by-day activities. It is good to share of yourself and the notable things that you experience, just don’t overdo it.
* Negative thoughts. If you’re feeling down, it’s usually better to keep it to yourself. People are attracted to positive attitudes rather than negative ones. Of course, if you do want to reach for connection at a time of need, Twitter can be invaluable.
* Extended conversations with individuals. Anything much more than a couple of to-and-fro tweets is stultifying to everyone else. Move to direct messages or email.
* Old news. Don’t share things that everyone has already seen. If you’ve seen something on the TV news, be sure that everyone on Twitter knew about it a long time ago.