Hey Motobabble-
Disclaimer: I used to manage ad sales & operations at CarDomain.com, the largest social network in the automotive space.
I browsed your site but didn't create an account, create my own car page, or upload any pics.
Your site looks great: very nice photo browser, look/feel. Some hopefully constructive criticism:
a) having to login to read comments posted on a vehicle is a major barrier to users spending time on your site, and is going to limit the SEO value of your content => why not just show what people have to say, and provide more engaging content to the folks who happen to find your site via search? By showing this I think you'll increase both your SEO and likelihood of converting browsers => registered members.
b) what's the value of membership? what's your "club?" CarDomain, while it doesn't have the glossy look/feel of your site, has a very large membership (850k members/cars profiled last i checked), and vibrant sub-communities of conversation around trucks, donks/boxs/bubbles, tuners, etc etc -- what's your community, your niche? who are you catering to? where's your beachhead? You may want to consider catering to a specific community to help increase your traction, and expand outward once you're rolling...
c) are you going to police your content? browsed and noticed a couple fake-y looking profiles, e.g. http://motobabble.com/bearx/2012-a4 which is listed as an Audi (i <3 audis btw) & it's actually stock/OEM photos of a VW... if you're building a community they'll appreciate accuracy & authenticity. The blog staff @ CarDomain invests a lot of time doing this, and their community really appreciates it... nice to know if you ask someone about how their Bugatti Veyron handles that they actually know.
As @dif points out
i) one reason CarDomain & others remain so strong is the amazing amount of content and membership they already have. How can you quickly build a community? Hmmm, if only you could... be the car-talk/car-photo-share app on Facebook... integrate with some existing auto sites to make photos "easier"... many, many angles to pursue.
ii) foster / cultivate specific communities: truck folks like trucks, Chevy/Ford truck folks like similar, and Scion folks will like to converse with Scion folks, etc etc... not that they won't talk to each other, but birds of a feather def'ly flock together. Perhaps this ties to the 'beachhead' point above.
And actually, reach out to CarDomain/Streetfire -- they're great folks. They'll be difficult to steal traffic from too because they're amazingly good at building fostering their own community, but I'd be surprised if they weren't pretty cool frien-e-mies to talk to.
Good luck-
--JeffC