I recently upgraded from 1.5.2 to 2.3.1. That all went incredibly smoothly. I had no problems with the default theme or the old theme. I switched themes and now it seems that certain links are not recognized by style.css which is indicated by a:hover not working on those links.
This problem only occurs in Mozilla Seamonkey and Firefox. It does not occur in IE7.
You can see this happening at my site http://bevilacqua.us when you mouse over the latest post header or the 'Hop You Like a Hurricane' or 'Music' tags in the older posts.