The single most disappointing application that came installed on my T-Mobile G1 was the default email application. The very first thing I noticed was the horrendous IMAP support. I have a very well organized mail account. I might have a have dozen or so folders at the top of my folder hierarchy. The Android email application just flattens out all of my 100+ sub folders out into a huge, difficult to navigate list.
K-9 still flattens out my folder list but lets me choose which folders I would like to display and sync. Fortunately, I really only want quick access to five or six folders while I'm on the go.
The beta release of K-9 supports the IMAP IDLE feature. This allows it to stay connected to the IMAP server so that it can instantly be notified when a new message arrives. This is much better for me than waiting up to 5 minutes to see a new message arrive.
Improved Message List
K-9's message list view is a huge improvement over the stock mail client. The view is much more condensed and looks like it fits about twice as many messages on the screen at the same time.
So far there is only one ChatterEmail feature that I miss. It had a summary view that displayed emails from multiple mailboxes on one screen in chronological order. Each message was color coded so that you could very easily tell which account each message belonged to. I saw a few mentions of ChatterEmail in the K-9 issue tracker, so I know I'm not the only one who misses this feature.