I think there are a number of "hard parts." Getting encryption right -- really right -- is always hard, and my "requirements" for full-text search and capabilit

I think there are a number of "hard parts." Getting encryption right -- really right -- is always hard, and my "requirements" for full-text search and capabilities-based sharing make that even harder. And I definitely agree with you that the caching implementation needs to be really good to work properly. Scalable tagging (database-backed FS design and mechanics) isn't as easy as it looks at first glance, either. But if I had to pick one hardest part, I'd vote for the UI. This stuff, at least the way I'm imagining it, is a significant departure from traditional file manipulation semantics. It's a file system plus the kind of user interactions that "web 2.0" and mobile apps have taught us all to expect. People won't use something that's not both really easy and elegantly designed. And there won't be enough adoption unless there's enough feature set coverage to appeal to lots of slightly different use cases. I mentioned Dropbox a number of times in the blog post because I think they've done a remarkably good job at both implementation and user experience. That's pretty cool!