Bill Gurley answers the question – Why Dropbox Is A Major Disruption:
Drew Houston and his team had taken a hard problem — file synchronization — and made it brain dead simple. Anyone that had used previous file synchronization programs, including Apple’s own iDisk, constantly encountered state problems. Modifications in one location would get out of synch with those in another, ruining the entire premise of seamless synchronization. It wasn’t that these other companies did not understand the problem, it was just that they could not execute on the solution. The Dropbox team solved this, which was a critical innovation.
Once you begin using Dropbox, you become more and more indifferent to the hardware you are using, as well as the operating system on that device. Dropbox commoditizes your devices and their OS, by being your “state” system in the sky. Storing credentials and configurations of devices, and even applications are natural next steps for this company. And the further they take it, the less dependent any user becomes of the physical machine (HW and SW) that is accessing that data (and state). Imagine the number of companies, as well as the previous paradigms, this threatens.