Well, compared to Mailbox and Paper, Carousel is simple. Dropbox is not as simple as Carousel itself. There are much more complex apps developed by single developers than Carousel, for example, and many are not even developed for profit. There's very little investment that Dropbox has to do to keep Carousel running, but they decided not to and compromise a lot more than they expect!
Dropbox itself is just a folder that syncs. Small and simple, right?
Carousel is just a camera roll that syncs. Small and simple, right?
There is an amazing amount of engineering and planning that goes into both products though.