Speaking of simplicity.
Isaac Hall, co-founder of Dropbox competitor Syncplicity, answered the Quora question "Why is Dropbox more popular than other tools with similar functionality?"
It's a wonderfully in-depth answer, encompassing PR companies, the press, and how to structure a beta, but despite all of those influences it came down to this:
In the end, it really came down to one incredibly genius idea: Dropbox limited its feature set on purpose. It had one folder and that folder always synced without any issues -- it was magic. Syncplicity could sync every folder on your computer until you hit our quota. (Unfortunately, that feature was used to synchronize C:\Windows\ for dozens of users -- doh!) Our company had too many features and this created confusion amongst our customer base. This in turn led to enough customer support issues that we couldn't innovate on the product, we were too busy fixing things. […]
If you're starting a new company, the best thing you can do is keep your feature set small and focused. Do one thing as best as you possibly can. Your users will beg and beg for more functionality. They will tell you their problems and ask you to fix it. My philosophy is that they're right if their feature request is right only if it works for 80% of your customers. Until you have a lot of resources, stay focused on your core competency.
The time has arrived that three communities–the business, design, and technology communities–have independently discovered the same thing. That the best way to build new technology products, services, and the businesses that deliver them is to work in small, cross-functional, highly collaborative teams. To use lightweight, informal methods. To use rapid cycles of designing, making, and validating in order to test and learn and improve. To focus on the customer.