I agree that information on a forum is generally pretty low risk, but as I said in an earlier post, there is a little more at risk than one might immediately think.
- There are private messages between users, conversations which were thought to be somewhat private. Some PMs are only PMs to keep from derailing public threads (or are otherwise only relevant to specific people) and aren't actually all that sensitive, but some information or conversation may be mildly sensitive. Not financial or medical, but perhaps embarrassing in some way. Sometimes on forums people exchange contact info and even shipping addresses via PM.
- An attacker could impersonate a mod or admin and gain access to whatever private forums, messages, and reports that mod or admin has access to, plus the ability to randomly ban users, lock threads or delete posts that people may have put some time and effort into.
- An impersonator could edit a member's older posts, inserting nasty material that comes up in the Google searches of potential employers. The job candidate would probably never learn why they were denied the interview (and thus couldn't reply with "The site was hacked. I didn't write that."), and the mods and member never correct the posts because they're so old that they were edited without being noticed.
- As more and more large databases are breached and their passwords cracked, attackers learn more and more about what passwords people in general tend to use, and this strengthens their password lists and rule sets for other, completely unrelated attacks (source).
Again, I agree that forums are fairly low risk if the password isn't reused. This isn't a financial, medical, or social networking website, and I'm not saying to sound the alarm. I'm just pointing out a few additional concerns that people may not think about at first.