If you haven't already read my earlier post "Will the real Twitter please stand up?" you need to. But after reading that, there is a lesson that you (application developers) can learn. Twitter could learn plenty.
All one has to do is Google Twitter, and you will see that they are solely focused on fixing their scalability issues. I would argue that they should first put a "patch" on their usability issues. You only get one chance to impress, and I am certain that Twitter has likely lost over a million potential Twitter users who all voice the same thing; "I don't get it", "What do I do with it?" and "How would I use this?".
Unlike their scalability issue, this could be addressed easily, and quickly with a few introductory pages on their site. Tell the user the different ways they might benefit (remember, it's all about benefits, not features when marketing). Give them examples using different profiles ie - How a housewife might use it, How a blogger might use it, How a business might use it - Becasue especially with Twitter the user experience is differant for each.
Consider using screen shots in your tutorials, and even offering video. You can not do too much when it comes to first impressions, MAKE THEM COUNT!