Maybe I'll be horribly wrong, but it seems to me that in few years dropbox won't be relevant. (I currently use dropbox everyday and love it)
Back when apple was creating iCloud they offered dropbox a very large amount of money to be a significant part of the new service (I think it was around a billion $ offered?). Dropbox refused, which I respect, they want to pave their own way in the world, which they already have done to a certain extent.
However, since they started up pretty much everyone offers cloud storage natively. MS has Skydrive, Apple has iCloud, Google has Google Drive, etc. In the future I can't see to many people bothering to install dropbox when their device already has native support for a different cloud storage system.
I think dropbox does things the best currently, but it's potential customers are becoming narrower everyday. Mega just started up and his offering 50 GB for free. I've worked on my dropbox account for years (free version) and have grown it from 2 GB to 22.4 GB currently.
tl;dr: dropbox is a great service, but with every major company offering native cloud storage now, I can't see dropbox having nearly as many customers in the future, even if they do offer some of the best features.