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.
Isaac Hall (co-founder of Syncplicity) in an answer to the question: “Why is Dropbox more popular than other programs with similar functionality?” on Quora (this is just an excerpt, I really recommend reading the whole answer; the quote at the title is from that answer too).