The old Admin-SSL plugin stopped working because the cookie code was from the old wp-includes/pluggable.php. The cookies are different so the old plugin failed.
There is a replacement but it does not set the cookies for use with SSL pages only, which was a huge part of the old plugin. I’m going to see if I can get the old one working with the new auth cookie code. My PHP is horrible so I’m not feeling too optimistic.
Mostly WordPress 2.5 worked right out of the box for me. Since I’m using the translation plugin, I get lots of Mysql database timeouts. With 2.3.x I implemented this fix and the translation pages have no issues with database timeouts. The fix has been updated for 2.5 so my database is not complaining anymore.
Thanks to 2.5 I was able to get rid of 2 fix plugins, Optimal Titles and Full Feed. Both are no longer needed. I did modify Bad Behavior slightly and added the Remove Max Width plugin but that’s mostly it.