One thing I really liked about WP 2.3.x was being able to rewrite post-slugs. I've put WP 2.5 on my test blog and I can't see a way to do it. The handy little box is gone. I haven't found another way to do it.
As other people have mentioned, I also liked the flexibility of being able to move things around in the write-post section. Some things I never use, so they're at the bottom. It's a lot easier. It's like WP 2.5 went backwards here.
I think there's a lot of good stuff in 2.5 and look forward to using it, but until the write page is fixed I'm not downgrading my blog.