On that note, I would definitely work on the homepage a bit. Add in some pictures of people doing outdoors things (rock climbing, hiking, boating, etc.) Get people excited about doing something outdoors. I get that you're going for simplicity here, so obviously accentuate the search box (maybe make it and the search button a little bigger). Also, more contrast with the nav bar and background...green on green doesn't really set off the most important elements on the page (logo/brand and main navigation). Maybe also list further down who this might interest (hikers, backpackers, kayakers, etc.) and tell why. It seems obvious to you, but if you call out "Find Great Places to Hike", and I'm a rock climber, I may just bounce.
Subnav on the homepage: Recent Reports means nothing to me. Featured Content, a little more interesting, but still not the first thing I'd click on (if I click at all), Park Index is getting better (I can equate hikes to parks) and finally About Trailbehind, which is probably what I am going to click on if I don't immediately do a search. Move that up the priority list, and downplay some of the other stuff that isn't relevant to first time visitors (or make it clear why it is relevant).
Login: I know it seems logical that if you want to register and can't find where to, then you would click login. Don't make users do that if you've already convinced them to register (which you need to do more of...I still don't know why I would) Make the registration link over obvious if you're looking for visitors to register, and make it clear what benefit there is to registering (can you save locations, favorites, link up with other outdoors people, etc.)
Sorry, not trying to be overly critical here at all. I really like the idea and what you guys have built so far. We've just found with Anyvite that the more feedback we get, the better grasp we have on what needs to be changed. Iterate rapidly, right? Anyway, best of luck with things.