Looking at the list of updates, I'm irritated that long file name support is going to require another $40-$50 upgrade.

As for useful upgrades, I'd prefer if they could lose the address book correctly and just integrate with the OS X address book. I've had many problems with the 'synchronization' of the address books.

I don't use Spamsieve or other spam filtering. I have a single filter that sends HTML to a junk folder, and that takes care of like 99% of my spam.

Besides that, I find PowerMail much more flexible and powerful then Mail.app.