I hate browser-specific bookmarks that don’t follow me.
I do not want to have to use FireFox’s craptastic bookmark features (or that stupid star) – the Del.icio.us add-on can suppress the built-in features (but doesn’t require their suppression)
Other options for “syncing” bookmarks focus too much on the “sync” aspect and not enough on remaining stable and usable, they get in the way
I love that I can use it from any machine, running any browser and can add via web (no plugin or add-on required)
Bookmark “sending” – I can bookmark for myself or for a friend.
Not overly “social” – I can send my friends bookmarks, and they can send me bookmarks, but I do not have to care that my friend bookmarked a certain page, or that 3 of my friends bookmarked something and have it suggested that I do as well.
PRIVACY! – I can have my bookmarks open to everyone (helpful if I mention that I bookmarked something, they can go find it), or keep private things private. I can select either. Even make something private while sharing it with another person.
I am used to it – I know the workflow, I can make it work for me as I need it.
Tags – tags aren’t required, but are useful in sorting my bookmarks. Unlike folders, tags fall down the tree when they are unused, meaning I don’t have to look at 200 folders, and I never have to clean up.
Bookmarks forever – In a structured bookmark system, sometimes bookmarks would need to be deleted to make space. In Del.icio.us bookmarks are chronological, no need to delete, but yet I can go diving for bookmarks from last year with ease.