I noticed that my WP 2.5, compared to WP 2.3, has a very very huge database usage.
In average, a "mysqladmin processlist" will return 85 open threads with my database, and that was much less with WP 2.3
That may not look important, but with my blog hitting 30k unique users per day, I am already using half the resources allowed from my sql server, and some plugins can't work anymore.
For instance any of the "archives" plugins I tested (like KGarchives for instance, from katesgasys.com), when they're of the kind that tries to make a list of every blog note ever published in a single page (in my case, that's a bit more than 700 blog notes, dispatched between a dozen categories), fails to work, and loads forever without opening anything.
And yet, they perfectly worked with wordpress 2.3.
A simple test, a dump of the database on a private test blog where nobody connects to, has shown that these plugins work... as long as nobody is connecting to the database except me ;)
That may not be an issue for small blogs, but for big ones, that's a real problem, i'll be forced to downgrade soon.
Honestly I don't think that could be solved easily on my side, so at least, I can report it here :) - and of course, if you have had that problem too and have come up with a patch, i'll be interested :D