Cool features IMO are usually a reward for users to keep them happy rather than an attraction.
Example, the awards system is awesome for a user and always fun to have one on your account, however I wouldn't see anyone specifically joining a website to gain an award.
So, really, if you're getting the hits it's more a manner of (besides getting more hits
) creating active, interesting content, getting others involved in the discussion, and then eventually more people will see that topic and register to say something in it. Keep it unique as a forum, original as content(Unless a discussion of a recent happening), and always welcoming of new members.
If you have current members, contests are fun to make them post more, encourage others to join, and also pushing people to join due to the topic at hand.
Other than that, you really just need to constantly have it in the back of your mind to think of new ideas, and then eventually it will come to you. Sometimes on my new forum I'll have like 5-6 people on a day...next thing I know I have 15-20 people on at anytime.
It'll fluctuate and get discouraging but if it's truly in your heart that you want it, it will happen. Just keep it original, and good luck.
Brandon