I think this kind of idea should leverage wordpress.org profiles HEAVILY -- which of course would argue for it migrating to core over time.

For a while now, I've wanted to mark plugins and/or themes as "favorites" in my wordpress.org profile. That would really help me track down all the plugins I have tried, especially those I've used for different projects. Taking that idea a couple steps further, you could use those favorites as the basis for a "custom install" profile, automatically downloading and installing the latest (compatible) versions of the plugins for use with a new install!

Automattic seems like they might be on the verge of trying to use the .org profiles for something in future -- I hope it's something really useful like this!