You should always assume the worst. However, most forums however only gather small amounts of personal info (name, email, location) which on its own is not enough to identify you. So it would seem the risk is low.
However its not. The real problem is if you've used the same username, email address and password on this forum as you do on other forums, social networking etc. Then the risk becomes rather high.
This is called "password reuse" and its a very serious computer security problem. People fall into this trap because its convenient to remember only one password.
A typical scenario is this: A person uses the same email address, username and password across many small sites like the other forum and also for big sites like facebook. Get those details from 1 weak site and you can use them on all the others.
They target small sites (like the other forum you're on) because they are easy targets compared to multi-billion-dollar corporations like facebook who spend big on security.
So, hackers will target low-security forum sites (like the forum you were on) to get the usernames, email address and passwords of the user base. They then use those details to access accounts on much higher security sites, like facebook etc. Once they are into your facebook they might post something offensive on your wall or they might go snooping in your messages. Lets say they find a message between you and a friend detailing your bank account number. They can work out what bank that is and along with other details from facebook (like where you live, work, your bitthday, postcode, mobile phone etc) they could try and easily impersonate you with that bank in the hope of accessing your bank account. Or they could sell your details to others.
Alternatively, if you are using a gmail or hotmail account on these forums and the password to that mail account is the same in the forums, then the hacker can now log into your mail account. They could then use this position to garner further info about you (such as emailed bank statements), change your password on other services like facebook and take control of those accounts or just be a pest in some other way. When you hear about Hollywood stars having their twitter/facebook accounts hacked, this is usually how it happens.
What do now? Log into all the other sites that you use the same username or email address on and change your password. Do it right now.
Always make sure sites like facebook, linkedin, twitter use different passwords.
Always make sure online mail accounts use different passwords again.
Always make sure banking sites use different passwords again.
