Dropbox was a YC company, which meant they had quite a bit of free press and access to a lot of valley connections. I would imagine that had a lot to do with their success.
I'm sure it didn't hurt them but their execution to date has been impeccable.
One thing has popped up recently: Dropbox relies on the urls to folders in 'boxes' that are shared to remain secret, nothing stops google from indexing the contents once the location is known, there is no 'robots.txt' that instructs google not to peek there.
This could lead to a lot of people being highly surprised that the contents of their secret stash are suddenly open to the portion of the population that uses google.
The problem is that it's not just the users that nominally own the data that control this, basically everybody that you share that url with can 'leak' it to google, either by using a toolbar or by posting it on some webpage. After that it's fair game.
we had issues getting the press excited at launch. We built a fantastic Windows client. 3 years ago, everyone was running Windows. We were so excited to show the press, yet they all had Macs. Walt Mossberg wouldn't write about our product because it was PC only. Months after we hired our PR agency, we found out that they had never even used our product... because they too only had Macs. It's pretty hard to pitch a service when you haven't used it.*
I'm surprised: I could see a fantastic Windows client being okay before 2004 or so (this might be because I got a PowerBook in 2004, but roll with it). By three years ago the return of the Mac was obvious to pretty much everybody, especially people who care about computing. The people who're running tech blogs, writing forum posts, and so on are disproportionately Mac users.
One of the quora comments notes the difference between B2C and B2B.
B2B needs lots of features, a great sales team, and can be Windows only.
B2C needs a great simple UI (which may one day have lots of power features tucked away, hopefullly after A/B testing), lots of press, and multi-platform is a big plus.