I think it would be cool if someone created a plugin called Postrank.
The basic functionality would be similar to Pagerank by Google, but would rather be based on the total amount of comments the post has. Giving the options of setting a standard level of what would be a Postrank 5 or a Postrank 10 (ie a postrank 5 would have 500 comments or a Postrank3 would have 30 comments), whatever it may be, have a formula that you can change to have a better standard for Postrank. Then have the function show_postrank(); and let the postrank be displayed based on the total amount of comments or whatever the basis may be.
Just an idea that I think would make for a great plugin.