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For me it was mainly the small things. Numerous bugs in the interface (GNOME shutdown button would become distorted and stop working, KDE 4 was initally disastrous) drove me crazy. Then there's the way they seem determined to stick to the release schedule, ready or not. Running the regular apt-get update led more and more to application bugs.
Switched to Debian, no such problems. Now I'm not plagued by constant annoyances, and my OS problems are only ever significant.