I've been having some Windows98 flashbacks ever since I'm using Ubuntu 6.06 on a Dell C840.
There are various reasons: resume kills the sound system, sometimes the ethernet and once in a while corrupts the video (easily fixed by ctrl+alt+F1 ctr+alt+F7). The expected hardware support problems under Linux.
Since I've installed Google Desktop Linux I also had for a while sudden Firefox crashes (fixed by disabling their extension). Boy was that fun ! Not to mention Firefox 1.5 doesn't save the session (make sure to install the SessionSaver extension).
The latest culprit is OpenOffice.org. Usually it works ok, but then you have the random crash. It's nice to have a Crash Recovery dialog and all, but I would rather not see that as often. It also has the tendency (usually after some crash recovery) to not start at all. You just click on and odt file and wait... and wait.. and nothing. You click again: and wait ...
The solution is usually a restart (so un-linuxy).
But, all in all, it's a usable desktop that kind-of forces you to do something productive since it's so simple. With no fancy GUI to drool at, you're forced to think and work. Or, post in your blog about how productive you could be.