I believe one very important thing was the courage to put fairly high(1) price tag for the service and compete on the quality of the user experience and not on price.

People tend to compare the Dropbox price to for example S3 storage price, but I think that is wrong. I'm for example using the Dropbox for certain project files and I'm sure that at least in my case the major cost is not the actual storage, but bandwidth and processing.

Edit: (1) Compared to other file storage services.