Rands in Repose on just what makes Dropbox so special:
The magic of Dropbox is that it doesn’t ask you to think about what you do. You care about one thing: do I have access to the most recent version of my files? And with Dropbox, yes, you do. Wherever you are, so are your files.
I’ve been espousing Dropbox for a while now, but this is a well-articulated macro view.