Take, for example the BBC News site. Theres an awfull lot of content on there, some of which I would be interested in the thoughts of others on. Currently only certain, carefully chosen stories feature comments. diigo, however, allows you to comment on these stories, furthermore you can highlight actual pieces of text within the story and comment on them. And then others can view your comments and add their own. It is possible to have a linear conversation based around single web pages or even paragraphs of content.
When you take the potential of the above and add in a competant social bookmarking service you can begin to see where diigo is heading as a service. The diigo team are aware that there are numerous other services out there, the one that is certainly a huge obstacle in terms of social bookmarking is del.icio.us, a service that I absolutely love. To make the transition a bit easier you can import your bookmarks into diigo from del.icio.us, you can also automatically add your diigo bookmarks to del.icio.us