This sounds all great but how do you ensure the comments are correct and valid? Malicious organizations and individuals could use it to pollute the network with false information!

And where do you store the comments? Neither you as the author of a comment nor the hosts of the file are trustworthy. Both ends can manipulate their clients and promise heaven on earth. We need an 3rd entity! A single entity is maybe trustworthy but cannot handle the load of an entire P2P network. One solution is to use the P2P network itself as an entity which is "tricky" (comment and file must be linked somehow, the data must be distributed equally in the network so that nobody is flooded with comments, make sure nobody can comment a file twice etc. etc.) and to be honest I dunno if I and others are interested in wasting CPU cycles for your comments!?

Anyway, checkout this:

http://www.cs.cornell.edu/People/egs/credence