Yeah, I've been wanting a similar chrome extension, actually. HN comments serve as a very valuable filter for information that I learn from, and I think it'd be great if the rest of the web benefitted from that kind of proofreading. Little annotations as a layer on top of the web.

Managing the community would be a bit tough, but if you seeded it with the HN community, and managed to keep the same self-regulating culture, I think that would be a strong base to start from.