Also to note, Dropbox was likely inspired by the open source Unison file-synchronization tool. So they have to handle all of the shortcomings and caveats that Unison does not handle out of the box:

Caveats: https://www.cis.upenn.edu/~bcpierce/unison/download/releases...

Symbolic links: https://www.cis.upenn.edu/~bcpierce/unison/download/releases...

Permissions: https://www.cis.upenn.edu/~bcpierce/unison/download/releases...

Cross platform issues (unsupported characters across operating systems and file systems): https://www.cis.upenn.edu/~bcpierce/unison/download/releases...

Edit: Inspired by the the Unison Spec (published on 2004) which may be why we have conflict files etc. when using Dropbox http://www.cis.upenn.edu/~bcpierce/papers/unisonspec.pdf