We have had quite a few discussions on how to gauge the quality of research papers and academic journals. The consensus is that it is tough to identify a single metric which could reveal this quality factor.

I had an idea in this context. In the past newspapers used to have a single "Letters to Editor" section to which comments on all articles were sent. With the advent of the Net, we have a comments section under every article. 

* Similarly, why don't reputable journals introduce a comment space for their published papers, where fellow researchers could appreciate/criticise/query the works?
* What hinders us from having a reputation system for published papers - something where registered users could vote based on their perceived utility?