Ask HN: Building a portal with features supporting community participation

What tools, platform would you recommend if someone were to build a portal ground up with the following features:

a) Ability for users to upload (or share, link) user generated content (say, audio, video, images etc)

b) Ability for users to comment, rate and provide feedback on user generated content

c) Ability for portal owner/ admin to do analytics (page views, visitors from different countries, downloads etc)

I don't want to write everything from the scratch but instead want to leverage existing tools/platforms as much as possible.

Thanks!