On first blush, it's underwhelming. A lot of monotone flatness, which I'm sure hit some kind of trend that got it greenlit, but doesn't make for good UX.
The interface is a solid wall of text that makes it very hard to distinguish one link from the next. There's no signposts to make it easy to tell where you are, or what you're able to do.
I ran through the bookmarking process, and it's clunky. Still asks you to tag things, only now things like the suggested tags are gone, meaning you have to think even more about what you're doing as you save bookmarks.
There's nothing really new that I can see, just a coat of paint and a lot of gratuitous flatness. Flat for flat's sake is definitely this year's skeuomorphism.
Full disclosure: I work with a clipping service that might be considered a Delicious competitor. (But I'd have the same critiques even if I didn't).