I have a fairly successful (400k visits/month, 1.2m PV/month) site which features a directory of lyrics-style content. The bulk of the content is not unique and exists in many places across the web.
I figured adding commenting would help add more bulk to each page and give some long-tail on-page KWs. The comments are non-AJAX and Google is crawling the information. There's 10k comments across the site.
I have some automated filters to remove profanity and junk but otherwise comments are published in real time. Because of the niche I target, the quality of spelling/grammar is very low. I don't have any pagination at the moment so some pages are now quite long and the quantity of UGC content is considerably larger than the main content of the page.
Traffic went down post-Panda and has picked up slowly over time - almost back at normal levels. But I feel I could better manage comments and that they do have an impact on SEO.
Do these comments have value? Should I use AJAX to stop crawling? Implement pagination to limit page length? Use plus/minus ranking to give prominence to the better comments?
Of course, commenting is a good idea. Besides, making the page uniquely content rich and variety, it will also increase user interaction and make your site user friendly and give people some reason to come back. Just be sure to moderate the comments carefully and regularly. Be sure to use captcha for all posts to keep bot posting to minimum. Though you will still get tons of spammy comments for SEO links.