I think the main unsolved problem is that we need our notes organized both based on subject matter, as well as based on importance, with a high level of granula

I think the main unsolved problem is that we need our notes organized both based on subject matter, as well as based on importance, with a high level of granularity on both dimensions (i.e. sentence level granularity) It's very hard to come up with a good, simple mechanism that lets you look at notes from both of those perspectives and to mix those two levels (i.e. "show me the high importance notes organized by subject") One associated challenge is that we don't really have a good way of knowing importance of information ahead of time, so an additional problem is to come up with good heuristics for estimating importance based on other factors.