The pattern I've seen over the years is that a bookmarking site will arise, based on some fairly specific idea of what managing bookmarks should look like. As it catches on, the owners will discover that people have distinct and curious ways they like to manage their bookmarks. In an effort to grow the site, they will add features that try to please these different constituencies, and eventually end up with a kind of insipid and aggressively social product that no longer appeals to the initial audience, and resembles a lot of other sites.
At that point people move on to something else, and the cycle begins again. Sites that break out of it (like Instapaper or Pinterest) seem to succeed by picking a very specific vision of bookmarking and sticking to it.
I think of this as "abstraction syndrome", where you are tempted at every step to build a more abstract version of the product because of the diversity of uses people put it to. But that very process makes the project generic, boring, and less useful than the original niche idea.
I believe a similar dynamic explains why no one has "solved lists". It's like "solving social" or "solving writing". The details are where the fun is.