Accurate and up-to-date information is a must. The environment of the site is also important. If a website's users are hostile to somebody, they likely won't stick around long enough to contribute too much.
If people want to listen to music, they'll put a CD into their computer and website music blocks that. If you are going to include games, they should be tested for quality first. Free stuff sites tend to be pretty bad. If you can develop or find software to operate a service, such as a forum or fantasy sports league, you might attract users.
With forums, the primary things people consider before contributing are the software used, the look and the activity. Writing your own software increases your chances of success, but if you can't program, you should use IPB or vBulletin. vBulletin should be the software of choice because many people still think of IPB as free software. A PHPBB or lower software will turn visitors away unless you've got an established community, just like a Proboards gets ignored. A custom skin is required and not one that is on half the net's forums. This stuff does not mean that much, but it just shows that the ownership is willing to work hard. The activity is 1 of the biggest paradoxes. People won't join because there are only 5 members but it can never improve unless there are more members. To grow activity, the staff members and first members must post alot.
In general, content is most important, but the softwares and the look are very important, too. Gimmicks don't help your website, despite what people might assume, and they might hurt it.