I remember seeing this idea some years ago. I think it was called qrxiv.org or something like that, but can't find it anymore. I hope this one has better luck, getting the users in the fragmented space of preprints can be a challenge.
There's also https://scirate.com/ which occasionally has active discussions but, at least in my field, there's far from critical mass, and discussions only happen when someone kick starts and advertises a thread.
I believe the most active field is quantum information, which has enough activity such that paper get dozens of upvotes, but the conversation level is basically as you describe.
