I picked Rails because
a) I knew it
b) It is fairly tailored for this kind of site
But as others have said, you can make a site like this with pretty much any language/framework. What you do not want to do is make all the features right off the bat. You should build the simplest version to start and then feel for what you want to do next.
By the way, we're shooting for a mid-July open source release so you can see any details of our site you like at that point. It'll be a while after that til you will safely be able to stand up your own instance, but that will eventually be a possibility.
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