Dropbox is not another file sharing software; the application form to YC doesn't even mention file sharing:
http://dl.dropbox.com/u/27532820/app.html
Dropbox isn't even about cloud storage.
Dropbox synchronizes your files between your different computers -- silently, automatically, without you doing anything except turn those machines on.
Nobody did this before Dropbox and still nobody is doing it now (except maybe AeroFS, which is much more difficult to setup and use -- but certainly not iCloud or any other "solution" that is restricted to one OS or company, and certainly no backup solution either).
That's why it was a fantastic idea... which has since been coupled with a brilliant execution, yes. But the idea itself was amazing.
That isn't to say Dropbox didn't blow them out of the water in regards to execution, but it wasn't an idea that was completely without precedent. Ideas rarely are, even if they seem like that in retrospect due to one company out executing everyone to an insane degree.
This part is important. The Microsoft service only synced to your other Microsoft things. iCloud only syncs to your Apple things. Dropbox syncs everywhere.
Before I used it DropBox was just another file sharing/syncing software. One of a whole stack of 'em that all seemed to suck in one way or another.
I'm sure that they all promised that they would be bringing cloud file storage to the masses as part of the initial pitch.... and I tried them all because, like you, I wanted this service before any of 'em existed.
Why did DropBox win and all of those others failed (or, at least, didn't succeed so wildly)? Why was it obvious to investors that DropBox was going to win, and the others "fail"?