The desktop or one of the included applets leaks memory just out of the box even on a fresh and clean install.

Yes, but this doesn't work reliably. Behind a proxy, it doesn't work at all.

Frequent need to reboot or kill the desktop process because of the memory leak.

If you use PAN for phone tethering:
DUMMY connections are not listed in the connection dialog, so maemo-pan does not work as easy as it should. Nokia still has to fix this trivial bug.

Still haven't found any... oh wait, the E-Mail client now lists IMAP subfolders!

Honestly, I wouldn't recommend Diablo yet...