Dropbox is incredibly easy and convenient to use. It's very simple, and that's what makes it great. It's simple enough for almost anyone to use. I've pointed tech-unsavvy friends and family at Dropbox with great success. It's just a folder on your computer that magically syncs to all your other computers too.
Now, there's nothing in that which justifies having a developer conference. I definitely do not understand that. But the Dropbox service itself deserves the praise it gets.
Nothing done at that level of scale is trivial. Also, the founders are in pretty rare company given the success of dropbox - definitely something to aspire to.