What I love about Delicious is that it makes it easy to save and organize bookmarks so they're accessible from any computer. I can tag different sites by course or text, and I like that things can be easily cross-referenced, as opposed to placed in just one folder.
The sharing/social dimension of Delicious is great too, at least in theory. I like the potential for a community of people with similar interests to collaborate in building a list of useful resources. It might be a great way for students to assist each other in finding resources that help them make sense of units or texts we're covering in class (although as soon as I write that, I'm imagining all the sites they'd bookmark that I don't want them even looking at, like Sparknotes and Clifs Notes, etc.). And it could certainly help me share info with colleagues as we're planning units.
But what I find extremely frustrating about the site is that it's hard to find that community without a lot of trial and error. The public profiles don't provide a whole lot of information, so it's impossible to search "high school English teachers" and find people that way. When I do come across a site that others have tagged, I don't know whether the users are students looking for essay ideas or teachers who think the material on the site is good for lesson planning. I can start figuring out how reliable or credible I consider other users by looking at the whole list of their bookmarks, but I really wish that process was streamlined a bit.
Furthermore, and it might make me seem like a nitpicker, but I hate that Delicious doesn't automatically open links in a new window. If you're doing a search for something, and you get a big list of things that might be relevant, isn't it common sense that you're going to check out more than one of the sites? I think having to use the back button is annoying, and that it's logical and useful in this context to facilitate easy comparison of sites by opening them in new windows. I recognize users do ultimately have control of this - I know to use the control button when I click on the link to arrange this for myself, but I think that should be the default.
So I like the bookmarking, but think the whole social part of the sharing is only mediocre.