43 Folders | GTD-style email in Thunderbird
This is something I have been sitting on for awhile. I finally started using this and it is a really cool way to handle your email if you are into Getting Things Done. The problem I have always had with GTD email is remembering to routinely scan the folders of email for Next Actions or Waiting. plus the shuffling between folders is a pain. This way everything is in the same folder, I can pull up the unhandled inbox items (all unlabeled) or just the waiting for, just the Next Actions, etc. Also, as mentioned, you can create a filter to include multiple labels so you can see all Next Actions and waiting.
The only down side, at least from my work, is that if you are color blind it would be tough to tell what is in what category an email is in since they color the email in the inbox to tell you what the category is. Also you still need to train the spam filter so unless you automatically delete email that is manually marked as spam (as I do) you need to mark the spam and then label it for delete.
I have found it useful to flip the delete filter on, cntrl-A to select all email, and delete. It is a quick way to move through the email.