Well, I am required by my job (I am a computer forensics specialist for a Canadian LE agency) to monitor these issues and most of what people are complaining about is caused by a couple of things:
1 People are not letting the install scripts finish , " It sat there running the 'beachball for 20 minutes so I re-booted" . The installer scripts are set up to clean out a lot of old logs and cruft from the previous OS so this can take time if you haven't been running the cron maintenance scripts as required. (These are automatic on a daily basis but the machine has to be on at usually 3AM, to do the maintenance scripts).
2 People are leaving periphrial devices plugged in while they do the upgrade/install. This is going to confuse a function that probes the hardware to determine what kernel extensions may be required. You should never do a major OS upgrade with other hardware attached to a machine.