But nothing about broken themes and no warning that you might end up with a mess.
Hmm...so any time you get a major upgrade of something you've used for a while, you *never* expect it to change how certain things work for a bit? Case in point, IE7. If you've designed your site's layout for IE6, imagine how messed up it looks in IE7 right now.
Would you have expected that? Hell yes. It's a major upgrade, and IE finally listened to the vast majority of developers out there, and finally made it so that it functions better (not completely, ugh, but better) with web standards. To do this, they had to make some things in older versions *not* work anymore.
It's the same for *any* major upgrade. And with WP 2.1, I have to say, it seems to me the guys looked at many complaints, requests, issues and other things and implemented them in this new version. I was *happily* surprised the other day when I got to play with the new WYSIWYG editor - and it's *nice* - it's not a screwed up as the 2.0 versions. Many of the hacks I had to implement to get exactly what I wanted are now fully integrated into the system, and I don't need them anymore.
To expect a *major* upgrade to have some issues with older code is not a wise thing to think. Just like people who created sites to be perfect in IE6, and are now finding them completely messed up in IE7 - you have to take into account that many themes and plugins were made for an older version that has been *improved*, and some things may not work as they did before. You can bet your butt that the good ones are already working towards an upgrade for the new release so you can still have them.
But again, you shouldn't expect old stuff to work *exactly* how it did with seriously new stuff. You *should* expect a few surprises - which is why (as someone else said) you should try it out *before* you implement it on, say, a live site running your business. That way you can mess with things until they are how they should be. And so far, I haven't had many issues at all. A couple of unexpected things, but they were quickly ironed out (and I didn't get my panties in a wad about it, either).