Here's a quick list of things I think are worth considering....
1. Is there other forum "competition" in the industry/area your forum would cover?
2. Do you have any kind of existing community -- a mailing list, a popular blog that invites public comments, etc. -- to help get early traffic into your forum?
3. Is the content that your forum would cover something that people actually enjoy talking about?
4. Is there a larger web site that would be there to help promote and direct people to the forum for discussion?
5. Are there additional opportunities to market/promote your forum?
6. If the forum is a success, are you prepared to manage it yourself or find people to help? Will you be able to find people to help?
7. Are you able to handle the tech support issues that will come up?
8. (if applicable) Are you prepared to deal with content management issues (spam, unsolicited advertising, posts critical of your products/services/company, troublemakers, not to mention libel, slander, etc.)?
9. Is it a cost-effective addition to your existing web site, or if it's a standalone forum, can you justify the costs involved?
Probably think of more later.....