I feel like this could work out quite well, so long as they preserve the spirit of Twitter's original 140 char limit. That could be handled as simply as: The first 140 characters are shown (just like they are today), but there's a "see more" button/icon/etc. that expands the tweet to longer form.

That would seem to solve both issues -- short-and-sweet for those that want it, and longer as necessary.

(Note this idea has been baked for approximately 15 seconds, so I'm sure I'm missing quite a bit.)