I'm not part of WordPress, just a 3rd party hacker, but even so I think a couple of points should be made...
1. WordPress Codex says, "deactivate plugins before upgrading" (http://codex.wordpress.org/Upgrading_WordPress) and people who don't are really asking for trouble. Plugins are mini-applications that hook into WordPress and either change or build on WordPress core functionality. It is only logical that when the underlying core code changes then plugins can cause problems.
2. WordPress tells everyone to take backups before upgrading. These backups are important as many things can go wrong with an upgrade - even things that are outside of WordPress itself. Restoring from backup is relatively easy, as long as the backups exist.
3. WordPress, like all open source projects, relies on its community to test code. WP 2.8 had beta releases and a release candidate before it was finally released. Most of the issues that were raised during the testing phase were fixed before WordPress 2.8 was released but obviously everyone uses different server setups with different versions of PHP and MySQL so the few people who did test could not test on all platforms.
Which brings me to my final point - if everyone was prepared to test the beta's and release candidates and provide feedback to the developers, a lot of these problems would not occur.
Even opening or adding to tickets on the tracker is helpful. http://core.trac.wordpress.org/
All open source depends on users to feed information back to the developers. I share the frustration of everyone in this thread (perhaps even more frustration since I spent time testing the betas and RC1, with no problems until I put this onto live blogs) but, really, venting in the forum doesn't help. If we report the issues in the tracker and they don't get fixed, THEN we really have grounds for complaint ;)