It's thriving because it just works. Works when you're offline. Works if you try to save conflicting copies of a file. Works if you accidentally delete a file. Works if you need to retrieve an old version of a file. Works if your internet connection is spotty.
Rather than punting on the hard issues and creating another WebDAV-like service, they actually created something that just works.
I think this was pretty shallow analysis by the author, but to his very first point - I do think it's the elephant in the room for a lot of startups. One of the most consistent pieces of advice/feedback I've heard from serial entrepreneurs and VCs is to stay razor focus.
Multi-platform is key. Shared folders can be used to collaborate not just with people, but with Linux servers too. I let users send files to my site by sharing a folder with it. I predict a lot of services will do that in the future. It's an easy way to give people offline functionality.
It might work "awesomely" one some platforms, but the Nautilus integration is pretty buggy - it was a nightmare to try to get it working on my Ubunutu 09.04. I ended up having to just use the command line utility a user wrote.
Seriously. I left a comment on the host blog observing that points 2, 3, and 4 are basically the same, and can be summarized as: it's insanely easy to use. I set up Dropbox in about 20 seconds, and it did exactly what it said it would. Compared to every other synchronization tool I've ever used, Dropbox wins not by meters, but by miles.
Dropbox works fantastically well with Keepass, just save the .kdb file in your Dropbox folder. Open up Keepass on another system and there are all your password once you wenter the master password.
I'm probably preaching to the converted, I'm sure most people who frequent ycombinator know about that but I still thinks it's once of the best uses for it so far along with backing up critical files.
It's also one of the only online services where I actually am strongly considering subscribing and paying money to upgrade once I approach the 2GB limit.
And it couldn't have been easy. I can think of a dozen features I'd like to see added, but most of them would only appeal to all small subset of users and ALL of them would make the service more complicated.
Rather than punting on the hard issues and creating another WebDAV-like service, they actually created something that just works.