It would be nice if folders could be sorted not only alphabetically, but also by the number of new/unread/all messages. This would especially be useful for people with large and flat folder collections with filters that spread incoming mail all over the folders. To avoid scrolling, these users could then have their folders sorted by number of new mails and so have all relevant folders on top. The intuitive way to use this would be to simply click on the column title as in the message view. The main folders (inbox et c.) should always be on top for my taste. HThis could also be customizable, however I do not think it is important.
Yes, I'd love this, or maybe just the ability to sort subfolders of the inbox.