1. It’s more socialDiigo has an extra level of social networking that Delicious does not provide - at least not in a usable manner. You can connect with people that have similar interests based on what you tag.
2. AnnotationsThe annotations feature is very cool. When you bookmark something, you can highlight notable sections to refer to later. And any other Diigo users can see your highlights when they visit the page if they have the toolbar installed.
3. Superior UI and ExperienceAside from all the snazzy features, the core “bookmarks” interface is much better than that of delicious - offering many additional features and better organization.
4. MicrobloggingThe microblogging feature in delicious never got a chance. This is the “daily post” feature that basically posts a digest to your blog of all the bookmarks you have saved over X amount of time. Delicious always had it as an “experimental feature”, for 3 years. Diigo does it so much better, allowing you to post only specific tags to your blog as well as providing more customization features.
5. DiscoveryNow, this is something that delicious did fairly well but is pretty much a product of its large community. But Diigo does a great job at it too, allowing you discover what’s hot across the network but also within a group of friends. It also has a “watchlist” feature that allows you to keep tabs on certain tags in the network. And last, it shows you a river of bookmarks from your network - with a neat tag cloud to see what your community is tagging the most.
6. Better ToolboxYou can import, export. There are widgets, linkrolls, and tagrolls. They offer several ways to interact with the service - through context menu, toolbars, bookmarklets. There’s a Facebook app. You can “save elsewhere” too. So, if you still want to post stuff to delicious (let’s say you have a great community there), you can set that up. What this does is posts your new bookmarks to the other services whenever you post them to Diigo.
