I can think of things that I used to do on my premium membership...
* emails based on your username (
[email protected],
[email protected], etc). Could be real emails, or email forwards.
* blogs (authors can use them to list progress, or just overall blogs)
* secret forums (every forum has forums that only certain groups/people can access!)
or things other places have done...
* discounts on MMO merchandise (partnerships with other companies like allakhazam did)