I think that delicious is really awesome. I still use it pretty much every day, even though if anything the product has almost declined in quality over the past few years. It feels like Delicious could have been the start of something even larger than a bookmark service, it's my primary resource for finding specific content (eg: rails + tutorial, photo + howto, etc.) I'm aware that that is a fairly large technical ba…
+ Delicious is lagging behind in mobile support.
+ Delicious could be taking more advantage of the conversation that people are having in their Notes section.
+ Delicious could be taking advantage of the links that are shared between users: help me find users that have similar but unique interests as I do. This would be true social networking on Delicious!
+ Delicious places priority on what is "Hot". This is not terribly useful since a lot of "Hot" links have already made the rounds on Twitter or in my Google Reader first.
+ Delicious could be taking advantage of links that are not yet popular but have a strong interest in category niches.
+ Delicious could be taking advantage of a user's top 10 tags: it's not an accurate assessment of a user's interest, but it gives me a quick assessment if I want to subscribe to their feed.
(I wish I could make another Delicious, but I'm not a programmer!)