Jaunty Jackelope, compared to the Karmic Koala Beta I recently installed, seems to have been to Canonical what Windows Vista was to Microsoft. And both Jaunty and Vista suffered problems for the same reason; the OS's included too many new and "improved" features that hardware and software vendors just weren't ready for.
What really was the major issue with Jaunty was a newer version of the X server that eschewed the proven track record of using xorg.conf files for the auto-generated X configurations. Great idea but didn't work so well, especially for folks forced to use older graphics drivers or even anyone using ATI graphics cards and ATI's proprietary drivers. For myself, with an ATI card installed at home, it meant I had to do some rather tedious work arounds to get my graphics card working on Jaunty.
Another issue was the switch to Pulse audio being the default sound sub-system in Jaunty. Again, Pulse is great technology but people just weren't fully prepared for it. To this day I still have sound issues with my USB sound card "dongle" for my headphones, as well as issues with sound from Flash video in Firefox.
With Karmic, it looks as if the Ubuntu developers have not messed around with systems so dependant on outside support such as the graphics and sound systems. Massive improvements to the startup systems though have meant I have boot times of around 5 to 10 seconds (yes you read that right).
Jaunty was, however never an out and out disaster like Vista was for Microsoft. It had its issues, but with a 6 month release cycle, as opposed to Microsoft's years of development, things can get back on track very quickly, and Karmic sure seems to show that they are.
One of the big advantages of agile, open source development that Canonical has over Microsoft.