I was able to get the Visual Editor working again on my WordPress 2.8 version and thought I would share what worked for me. I tried most of the tips here (pages 1 - 2) and none of them restored the Visual Editor.
From the WordPress archives page, I re-downloaded a fresh copy of the zip/tar file and unpackaged it.
My thought was, perhaps my FTP client is corrupting (by not transferring file types in the correct format (ASCII or BINARY) or something of that nature. I was using GFTP on Linux. So, I used a different FTP program – FileZilla - still on Linux. As a note, I use Firefox (Iceweasel) as a browser with no extensions.
**Save a backup of your existing files before attempting any overwrites** To quickly save a backup, just rename the folder on your server to something like “wp-admin-backup” and then upload the wp-admin folder fresh.
I connected to my FTP site and for ALL the WordPress folders and files I completely overwrote all the existing files using FileZilla.
As a note, this next step may or may not be necessary – I also looked at the folder permissions and reset them - but exactly matching what they were. For example, if the file permission was 755, I used my FTP program to issue the “set to 755″ command again. I wanted to make sure that the folders were current with permission for the default domain FTP user I was logging in with.
Voila! I have the Visual Editor back!