First, I would like to point out that managing a forum is very different from blogging. Blogging can be done as a part-time one-man show. For example, you can decide to blog just once a week. And that's it. You just blog once a week. Then forget about it for the rest of the week, until it's time to write another blog post.
Managing a forum is a full-time job. All forums are global, which means members can be coming in and posting and commenting any time of the day or night, even when you are sleeping, even when you are at work, even when you are on holiday.
So what's the problem with that? Ah, lots and lots of problems. A forum cannot have a system where every post is moderated before it's published. That would kill the forum. You can do that for blog comments. Set all comments to be moderated before they are published and you will be fine. For a forum, posting is spontaneous. That means all kind of posts will get into the forum. Good, bad and ugly.
That's why managing a forum is a full-time job. You have to be around to keep an eye on things. If you can't be around all the time, you have to make sure that there's someone around when you aren't.
Is it worth it?
Well, if you are prepared to commit the time, money and effort, it should pay off. Eventually. I am on some forums which are so big that I can make a post, and in less than five minutes, my post would be buried under tons of newer posts.
Can you make money out of a forum? Sure. The simple rule of thumb for anything on the Web is just this: if you have a big enough audience or following, you can find a way to monetize it. See how Mark did it with Facebook?
No, no, don't think that you will become like Mark. That would be starting things off on the wrong foot. Start with what you enjoy doing. Mark didn't start Facebook to make billions. He started it just as a way for his friends to keep in touch.
You do the same, too. Let's say you are very keen on growing roses. So, start by blogging about roses. Then as your readership grow, consider diverting the comments to a forum.
Do you need to learn a new software to do a forum?
Not totally. If you are using Wordpress, you can consider using bbPress, which is a forum which works with Wordpress and is developed by the same people who created Wordpress. So the transition should be seamless, both for you and your readers.
If you are serious about starting a forum, I can point you to a forum which is dedicated to helping people who want to start their own forums. The forum owner has a lot of experience in developing forums and he is very happy to share his knowledge and experience. Of course, I am a member of that forum, too.