To forum or not to forum, that is the question. Many, many of my clients tell me they want a forum on their website. Not a problem! There are some great components out there for Joomla that make it very easy to start a forum on your site. I personally like Fireboard, but there's a new up-and-coming forum called Simplest Forum. I like simple stuff.
Here's the dilemma: Do you have the time and manpower to maintain the forum? Do you have a large enough audience to make your forum interesting? Are your topics of interest? Because you build it doesn't mean they'll come.
I've run my own forums in the past and I'll tell you the truth, they are a royal pain to maintain. Pardon the rhyme. Spammers are so sophisticated these days they have programs that crawl the web looking for free places they can post their adds for drugs, websites of ill repute and the like. When you run a forum you need to keep it cleaned up. You're visitors get enough of this junk in their email boxes and don't want to see it on forums.
Flame wars, FUD (Fear, Uncertainty, Dread) and Trolls. Sounds a bit like Lord of the Rings doesn't it? These are people with nothing better to do than cause trouble on forums. You have to make allowances for freedom of speech, simultaneously reining in the trolls. If your forum becomes acrimonious people will stop coming.
Forums are a great way to get people to your site. A by-product of an interesting forum is people will visit your other pages. You can also earn money from sponsored ads and increase your page rank by having people link to your forum. The bottom line is forums are work with reward. You have to make the call to determine if you have what it takes to maintain it.