If you just want to run a standalone web forum, then Google Wave may not offer much more than a ajax php forum. And if you just want email, then SMTP+IMAP/POP is surely good enough.
The power of Google Wave comes from the unification of various communication and collaboration paradigms, it's federated nature, it's extensibility and it's open-standard and web-centric approach. In the old model if I want to participate in a forum I'll have to register on the web, go back to my outlook to get the verification email, and then go back to web. I'll also have to subscribe to email alerts for new posts, then go back to the web to reply. All these context switching is totally unnecessary and can be frustrating when you have say 10 different web forums, 5 social networks, 3 photo sharing sites, and 2 IM networks and 1 blog. And there is no practical way to for me to search and browse through my entire communication history in one place.