I monitor a couple of other forums one of which has now been disabled by a hacker. I don't know how much damage was done, or whether the operation is designed to spread viruses amongst users. (Don't worry, I didn't log in !)
I've cleaned up about a dozen such cases, in each case the only danger to the end users was that the hack put malicious code in the forum, trying to get adware installed on the end users' computers.
However, there are additional vectors for attack, such as identity theft. If you use the same password/email on other sites your login credentials might be compromised.
For example, let's say you use the same email/password on the forum as you do for paypal, there is danger to you that by hacking the forum they can gain access to other sites you log into.
But do note that most forum hacks are automated and they don't go beyond defacement and attempts at drive-by adware installation (which a patched computer is almost always going to protect you from).