Lots of stuff!
While you're right that a regular wiki can do most (not all) of what OP offers, plus lots of stuff it doesn't, there are still plenty of reasons to use OP:
1) No setup - With OP, just create your campaign. No software to FTP to your server, no upgrading to the latest version every month, none of that.
2) GM-only info on every page. Any other wikis that have private and public stuff on the same page?
3) Adventure Log - It's a blog built right in, for tracking your adventure sessions.
4) NPC tracking - Manage your NPCs easily, keep some GM-only, and easily sort and filter based on tags.
There's even more I could go into. Yes, you could do much of this with a regular wiki, but then again, you could do it all with bare HTML as well. The point is, you don't *have* to. OP makes it easy on you.
Believe me, I tried all manner of wikis/blogs/whatevers and none did it right. So, I started building OP from scratch, and here we are today!