Nothing is perfect. Certainly not software as complex as an operating system. I made a post about the five best features of the new Ubuntu release, so now I'll be making a post about the worst. Worst for me at least.
So here's my list:
New boot experience - Ubuntu's target of a 10s boot is certainly a good thing. Unfortunately, it's an optional target for Karmic. Lucid Lynx is the one that's supposed to achieve that wonderful target for Ubuntu users. A bug in the new file prefetching daemon, sreadahead, caused a lot of users to experience boot time regressions.
Personally, my boot went from 19s in Jaunty to over a minute in Karmic. That's quite a disaster. On the other hand, it's still much better than Windows and it doesn't really feel all that slow, after all. I just wished it were blazingly quick like Jaunty.
Poor video playback performance - H.264 video playback seems to have improved some but there's still issues with high resolution HD content. With HD content getting more prevalent nowadays, it's not only imperative but necessary to have smooth playback. Hardware that aren't the latest and greatest but still capable of playing these formats should not be held back by the codec.
I still need to kill off FireFox sometimes when playing 720p or higher video. Otherwise, I can expect an annoying lag or stutters during playback. It's much better than in Jaunty but still...
Media collection management - I never can get myself to like Rhythmbox. Somehow it just screws up IDv3 tag handling. Despite changing it from within Rhythmbox itself the tags just refuse to 'stick'. When I change the IDv3 tags in Rhythmbox it'll stay for a few seconds then revert to the original. This happens with non-ASCII tags mainly. Not all files are affected. And I'm even trying to change Japanese or Chinese characters to English too.
Maybe it's just poor i18n support, or something in the software that I need to do but somehow didn't. Ubuntu is supposed to Just Work so I chalk it down to Rhythmbox's failure. There's also the fact that I can't do mass tagging or file renaming from inside Rhythmbox. Bad.
Files and folders sharing - File sharing on Windows networks hasn't been working at all for a long time. In fact it's quite a miracle for it to work at all. I can't open Windows shares from Ubuntu almost all the time. I can't access Ubuntu shares by hostname from Windows at all. Good thing I can still use IPs to access them but Windows users will not expect this behaviour.
Hardware detection - Karmic is the first release for a very long time that didn't properly set my sound card default preferences. I had completely forgotten I even had on-board audio until Karmic!
A clean installation of Karmic lead to the on-board audio device being set as the default output device. Not only that, it decided that 0% volume was the correct volume to set. I had to manually set the device to my sound card and change the volume levels. Looks like SoundBlasters still don't work very well with Linux in general.
Overall, Karmic has improved significantly. Unfortunately, there's still warts here and there that users will complain about. The occasional regression is regrettable but not completely unexpected for hardware drivers. Just look at the complaints on Windows 7's UAC. There's a fine balance between convenience and security. Finding that balance is the problem.