Having personally presided over an online community that never took off (in fact, wasn't able to taxi down the runway), I know how easy it is to make bad assumptions about how things are going to work when it's merely bits and bytes you're shifting around on your screen. Behavior is hard to predict, and key assumptions should be poked at, repeatedly, to test their validity. This process, partially is captured well in the recommended Three Questions to ask when considering an online community, from Forum One's excellent Online Community Report blog.