This is something I was thinking of doing a few months ago. I would be interested in contributing if you need or want the help. At the very least keep us updated!
I liked the suggestion by natch 27 for offline. It doesn't have an offline feature despite have a browser plugin (I am not a huge fan of this).
In terms of interface I don't have a complete answer. I want to be able to have my bookmarks organized but a lot of the time I don't want to do it manually. Too often I bookmark something really cool but don't have to time to tag or comment on it so it disappears until I decide to organize my bookmarks which in and of itself is a pain.
Maybe somehow generate metadata/tags/etc from scraping the link or by what other people have to saw about said link? Of course this would be in addition to being able to comment/tag/etc yourself. Thumbnails of the linked page would be nice, or if it was something from imgur for example, maybe the thumbnail is just the image that is on the page? I could go on but I think you might see where I am coming from; a lot of different features seem awesome but I think a lot of testing would be needed to see what people use and enjoy most.
* Sites I visit often and don't want to have to type to access
* Things I found and want to read (or reread) in the future
* and finally pictures or funny things I want to be able to share or look at later. So almost the same as #2.
For for items 2 & 3 offline access would be a great feature.