FIX THE GLITCHES!
New Yahoo mail format has too many bugs. Emails close while I'm reading them. They at one point would actually scroll through by themselves so I would never know what I have actually read and what I haven't. Tabbed messages close by themselves. If the new format didn't have all these really annoying glitches, it wouldn't be so bad.
If Yahoo's intention is to drive their users to Gmail, they're succeeding.