The social point is a good one. You can enable commenting, sharing, downloading, etc with OpenPhoto. So your comments live on your server.
In terms of discovery - I agree that isn't a fully solved problem. You can share a link to your photo and past it into Facebook / Twitter / etc. You can use RSS to follow someone, and search by tag etc.
But, yes, for now there's no way that I can find of seeing "all photos taken by users tagged with X".