A common and much disputed saying about online communities is 'build it and they will come,' referring to the concept that all you need to do to create a thriving online community is launch the software. Obviously, it's not that simple.
There's a lot that goes into the success or failure of an online community - strategy, site design, integration with whatever back-end database the org is using, etc. Not to mention community management. But one facet of online community that I don't think gets enough recognition is support, e.g. help for the person/people managing the community and helping them make their platform as good as it can be.