I successfully rolled back to 2.7.1 after experiencing numerous errors upgrading to 2.8. I'm neither a newbie nor super-experienced, but I'd made backups and I had 2.7.1 on my hard drive still from the last upgrade. This experience got me thinking about what I dislike in WordPress.
I have another blog in Textpattern, so I have some means of comparison--WordPress updates are so frequent (if I include the plugins that need to update periodically, etc) and can be so potentially devastating to a blog that I wonder why there's an "Automatic Upgrade" button at all--the forum posts are filled with newbies who've not backed up, and who expected the button to work right. It has never worked correctly for me and I've always had to manually upgrade and if there are going to be problems, it sure would be better to go slower and step-by-step than just to push a button and hope.
Secondly, there's no way of knowing when an upgrade has had all the kinks worked out. I assume that when the upgrade is posted, it is out of beta and good to go. Not so, apparently.
My problems had to do with the wp-admin and wp-includes folders. After upgrading manually, I still had error after error. At one point the blog loaded, but I couldn't get into the Admin page. I kept getting errors that lines were missing or classes couldn't be found. Perhaps this is a problem with my theme, but I can't tell. 2.7.1 works and I'm back to it for now.