Excellent summary. Regarding number 2, how do you think the context problem could be solved? The application can save the tab history of how you got to the page, but then you would need to install a plugin and possibly send navigation info to an online service. Or you could add a note to the saved bookmark, but now it requires some of your time.
I agree with you that user experience is important, but seeing so many alternatives that fail makes me wonder if maybe what is needed is a disruptive feature such offline access. Pinboard has everything you need to bookmark and is fast, pleople love it. However it does not have offline access to files and I wonder if that would make the difference for some people.
Thx for the answer. Let me deep some more. Every month comes up a new bookmarking service with a very cool interface but it never get adoption and last less than a year. What exactly would make the difference for you?