It seems like there are far more programmers creating back-end for this kind of thing than there are designers working on good concepts and front-ends.
I'd build it on top of WordPress as a plugin. Then most of the interface decisions are done, the community is already there, and installation takes about 3-4 clicks.
It seems like there are far more programmers creating back-end for this kind of thing than there are designers working on good concepts and front-ends.