I have hammered on this topic before, but as I keep talking to people about communities, I keep hearing them think about it the wrong way.
Here is how to think (and not to) about them:
1) We're having a problem with engagement in our community
Are you sure about that? How do you know?
Let me guess: you are looking at one place (url) where you expect your community to interact and you only see a 10% participation rate.
If it works, your community consists of a group of people who are together because they share a particular interest. If they do, then they may hang together at the URL you are expecting them to hang out or they may not. Think of any offline community you belong to - sometimes you congregate for big get togethers, but often times you have smaller sub-groups who get together in different places, or pairs talking with one another over the phone. Why is it that you expect this to be different in the virtual world?
The largest active group within your community is the active lurkers - that group is very active, but just not in the visible public space. A good portion of your community is also hanging out in other places, not just the URL where you are expecting them at.
2) Should I run my community on SharePoint or should I use some other technology?
Don't take me wrong - technology can make a difference. But it can only make a difference if you already have a successful community. Technology is not going to determine whether you can have a successful community. In fact, and if your community would not survive in a bulletin board, it will not survive anywhere.
3) I have an "addressable" community of a few thousand people - do I need to develop content?
Ok, so if you succeed you can expect a few hundred people to engage with you. What do you think will engage them? You?
Forget it!
They will engage along a common interest. If they do that, you better have some valuable content about that interest before they first show up. If you don't they will never come back. And even if you have thousands of potential contributors, you will need to develop content for them. It is very hard to develop communities that can sustain themselves on user-generated content - in fact those communities are extremely rare.
Think of your community's lifecycle
Modern tribes are nomadic by interest - and at some point people in your community will move on. Your community has a lifecycle, and you need to watch out for end-of-life situations. Just like off-line conversations, some will peter out. And just like brainstorming sessions, some need to be shut down after a certain time. When that happens you need to provide closure for your community - develop a white paper, a web site with the results/findings of the community, etc.
Participating in communities that are leveraged for business is not all that different than participating in communities in your personal life. You know how to manage your behavior or lead people in your personal life - apply those same principles when you are at work.
It's simple - Be Human!