Google Sidewiki allows anyone to annotate any web page. I think it’s been tried before, but this is Google. I first found out about it from a blog post by Frank O Dwyer, who points out that it will be very handy for commenting on blog posts on blogs where comments are suspected to be removed. Google are pitching it as a way for people to add useful information to web pages. That seems a good idea, too, especially when there is no other way to comment.
It could do with being a bit more feature-rich — threaded discussion doesn’t seem practical. Perhaps Google Sidewave would be a better idea.
Not everyone likes it. I don’t think I see a problem. It’s no different from a discussion forum where a new topic is created for every web page. It’s just that the interface is better.