I've done several interest searches and found a lot of inactive communities.

Yeah, my impression is that when open beta started, a lot of people just sort of copied the names of big LJ communities over and waited for a flood of people to materialize and start posting in the comm. And when that didn't happen, they abandoned the comm.

So there are a number of comms that died out after a few introductory posts. The comms that have taken off are the ones where the mods have been really involved and put a lot of effort into promoting the comm, starting posts, getting other people involved, etc..