Well I have been around the traps trying many different types of online environments from wikis to blogs to enterprise LMSs and more. The needs of each community will be different, however I am narrowing down my preferred tool.
The decision has been difficult since there are many competing feature choices:
Integration of features provided by enterprise systems compared with a need to mash together multiple websites
Price and flexibility of workspaces
Attractiveness of the site
Ease of use
Ability to facilitate social connections
In the end the latter element, the ability to promote social networking has become the determining factor, without that there will likely not be a community where particiaption is voluntary and the membership relatively inexperienced and unfamiliar with online community environments.
For a fully integrated system with good pricing schemes was CentralDesktop. It does a lot of things extremely well. The killer however was the cost and inflexible pricing scheme with multiple workspaces being available however the number of users for each workspace is restricted.
So my preference now is Ning , though Grou.ps follows closely behind.
Great social networking elements
Good forum, notifies by email when new discussion are added
Chat feature for instant messaging
Who is online feature
Can feature members and videos, e.g. someone has made valuable contributions recently, feature them on the front page.
RSS feeds updating with changing content on the site (and external RSS feeds); this makes the frontpage dynamic and up-to-date which is very important for encouraging frequent return visitors.
No file repository, though can integrate box.net
Wiki does not appear to be as fully functional as dedicated wikis
I think that this oculd be used to engage in very effective collaborative professional development
for teachers. It would be nice to see the ability to have a member blog feature so techers can reflect on their learning journey.
I have also explored Grou.ps and there does not seem to be a great deal different, except for a file repository and a few minor features.