I disagree that the barrier to compete is not very high. There have been "alternatives" for years - see FolderShare/SkyDrive. There are plenty of alternatives

I disagree that the barrier to compete is not very high. There have been "alternatives" for years - see FolderShare/SkyDrive. There are plenty of alternatives now, but the reality is that their implementation sucks. We tried using JungleDisk but it is just too inefficient. Others were just too fiddly. Dropbox is so easy that my wife and I use it to share files between computers at home rather than do local file shares. Seriously. And it was her idea. Dropbox is so far ahead of the competition it is crazy. They just need to keep that advantage.