I installed 2.5 on a new server and the TinyMCE editor didn't show up. After looking through the forums, I found someone (http://wordpress.org/support/topic/143030/page/2?replies=42) posting a fix to a file, /wp-includes/js/tinymce/tiny_mce_gzip.php, but when I went to open that file in 2.5, it was empty.
So, the TinyMCE wasn't showing up. When I downloaded 2.3.3 and installed it, I had no problems. This a bug with 2.5???