Earlier quoted context omitted.
> But I could duplicate the entire thing in about a day with inotify and rsync. The scaling is mostly on Amazon. No, you could duplicate a narrow piece of Dropbox's functionality in a day. You could not even get anywhere near duplicating the product and the service in anything less than months.
Dropbox pretty much uses librsync and, I believe, inotify on Linux. With Amazon S3. That is Dropbox. Literally. Polishing any product takes time. But that's universal and has nothing to do with engineering. What exactly is non-trivial about Dropbox?
In all seriousness, from a highly abstracted view, Dropbox isn't all that complicated. However, the same things are said about numerous other projects. I remember when Stack Overflow took off, and everyone around here said "I could build Stack Overflow in a weekend".
It turns out that, yes, you can build Stack Overflow in a weekend. However, all you've managed to build is a basic Q&A site. What you've missed is practically everything that makes Stack Overflow work. The rep count probably doesn't work as well, the moderator controls aren't there, the OpenID integration isn't there, the markdown controls aren't as sophisticated, it's slower, etc.
I don't have a link to it, but not long ago there was a repost of the original "Show HN" for Dropbox, where there were similar criticisms of how "trivial" Dropbox is to implement. The devil is in the details.