Plain-text and a file system? See emacs' org-mode. It has many of the features parent listed.
update I'd even add perhaps the solution is really to take the best ideas out of org-mode and give it a platform-integrated UI. It does basically what everyone here has been asking for and more... it's just buried in Emacs which is definitely not for everyone.
Document sharing across devices is easy if you're comfortable sharing your content with a third-party like Evernote or Dropbox. I'd be more comfortable with something that's decentralized, encrypted, and versioned like a Git + Freenet backing store.
Yeah, for a long time my wiki based file system based, and the markup is plain text, so you can read it without any software. (I've since switched to sqlite for query flexibility.)
I hear about org mode a lot when I talk about the Wiki, but I'm not an emacs user so it's not as appealing to me.
Can org mode export HTML? At least for reading, plain HTML is the format that works best across all devices, and gives fast access. Writing is a different story.
It can export to HTML, PDF via LaTeX, and a couple of others.
Emacs is definitely not the best editor for everyone. It's a shame that org-mode is sort of a secret weapon of Emacs. It's such a great package that I think it could stand on its own.
update I'd even add perhaps the solution is really to take the best ideas out of org-mode and give it a platform-integrated UI. It does basically what everyone here has been asking for and more... it's just buried in Emacs which is definitely not for everyone.
Document sharing across devices is easy if you're comfortable sharing your content with a third-party like Evernote or Dropbox. I'd be more comfortable with something that's decentralized, encrypted, and versioned like a Git + Freenet backing store.