1. Definitely go mobile
2. Detect my location and immediately show me stuff nearby (there was a geolocation database on HN recently)
3. You're making me zoom in too much with those nice aggregated bubbles, at some point I just got tired
4. Socialight built a nice community, but they don't seem like they're going anywhere (incidentally they're based in NY too!) - they do some good things, like mobile strategy but the interface is hard.
5. Don't force the sign up before someone adds content, let them add content and then ask them to sign up or provide an anonymous nickname
6. Placemarking and location based services is a massive but very hard and less understood market, keep innovating and develop based on feedback - community is very important!