My biggest issue with the way browsers do bookmarks currently is that they force them into a tree structure.
They're not trees and I hate that I have to put them into one. Tags and labels are where they should be, more like the way things like delicious had them. Even though Firefox has tags, they're not prominent and not the primary way of organizing them, so they're not as easy to use as they should be.
Of course each browser has extensions that provide things like what I want, but none polished enough to make integration seamless and make adding new bookmarks painless enough to use, at least so far as I've tried.
The other important thing for me is being able to clip portions of a page and being able to annotate a page with notes, references and comments.
For all of those reasons, I'm stuck using an even less tailored tool (Evernote) to do my bookmarks. So, here's to hoping that improving bookmarks ends up being the next frontier.
Oh and -- by the way, your email validation in the comments here is broken. It thinks +'s are not valid.