I upgraded to WordPress 2.1 last night. It went fairly smooth, but I broke a few things. 
My plugin that manages my Amazon items is now inoperable, meaning that my cute sidebar items saying what music I’m currently listening to, etc., are now gone. Also, my sections of recommended music, movies, etc. are gone, too. Oh well… at least now I don’t have to worry about updating them all the time. 
In terms of XHTML and CSS I had to rewrite some style to fix the layout of my theme. This custom-made theme I use employs very complex style markup and thus is very sensitive to change, and the upgrade to version 2.1 brought on some small syntax changes.
And then of course there was some PHP I had to rewrite… mostly because version 2.1 handles pages differently in the database schema.
And then I had to patch up my Jumps pages because version 2.1 totally handles link categories differently and also deprecated some primary link PHP functions.
Wow… I guess it was a lot of work now that I think about it.