I agree with hcf, it's the lack of commitment. I have seen forums open 1 day and 2-3 weeks later only have 6 posts and no activity.
You got to show the public you are committed to having the forums. Show your active around them, promote it and affiliate with related forums.
Having a premium domain like .com, .net etc are great in showing the public you have spent a little money on it. If your using a forum software, think about getting a custom skin. There are a few people and companies who will do it for an affordable price and this shows people you are dedicated.
Obviously there is alot more factors about how to make your forum stand out, but some of this may help.
All the best!