Preview? To gain critical mass around an area was our initial intention. By the way, we are not very restrictive at the moment, as long as you request, we will give you a preview invitation!
Downvote? To stay truthful to the real-world model, we hope to let things die down quietly. In the next version we will be implementing flagging, and maybe consider downvote as needed. This is a good point!
See more before joining? This is a chicken-egg problem for us now: without large adoption, our content is suboptimal so we are reluctant to open up the experience to everyone; but most people want to see more before contributing...
So at the current stage, we are hoping to target the early-adopter crowd (for example, who would wait 6 hours in the line to get the first iPad), then open it up! I would love to get some discussion around this if others would like to chime in!
Get location from IP? Good idea, we should give it a try! However, we probably will stick to browser's native support - I think this is cool and might be the future, no? :)