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From the start, Dropbox was almost magically simple: Install Dropbox’s folder on your desktop, and by simply dragging files into it you could suddenly access them from anywhere.That was simple in 2007, but this kind of synching model (also used by Google Drive and Microsoft OneDrive) doesn't feel simple anymore to me. Most applications still save files in other locations by default. Having to save them in your Dropbox, or having to move them to your Dropbox afterward, turns out to be a massive friction.
If the goal is that "Your whole computing environment ought to follow you around", you need to remove that friction. I know at least one person who lost access to some important files when she needed them because she forgot to drop them off in the right box.
One possibility would be to do what Microsoft does with Office 2014 and OneDrive, and try to force you to save all your files inside the synched folder. But that quickly gets annoying, especially since most people already have mountains of files stored and organized elsewhere.
That, and the lack of client-side encryption, is why I'm a loyal customer of a Dropbox competitor that allows me to sync any folder on any device with any other folder on any other device. I set it up to sync my entire $HOME partition, so I don't need to care where my apps store their files. That, Mr. Houston, is how you get me to hand over my "whole computing environment" to you.