Here are somecanuckchick‘s Top 5:

• Editing
  Editing for content—specifically corrections, or updates, to information contained in a recent tweet—would improve the quality of tweets.
• Filters
  By filtering tweets, information is directed accordingly. Filters—based on #hashtags—would allow for the selection of information by followers, and for the control and direction of information to followers.
• Threads
  Discussions often take place via Twitter. Threads would help to track an entire conversation between 2+ people.
• Hyper Link Text
  Hyper link text would increase the quantity and the quality of information in tweets containing URLs. (It would also make tweets look a lot cleaner!)
• Multiple Avatars
  Different types of tweets warrant different kinds of avatars. Avatars—based on #hashtags—would represent tweets containing specific information.

Is there a Twitter Feature you would like to see? Let somecanuckchick know!