[brianchu]

Some of the technical challenges I can think of:

1. Version control.

2. LAN sync.

3. Dropbox probably also does a lot to try and minimize their S3 costs - so probably a good deal of compression/duplication-detection on the backend.

4. Any and all networking issues. Even if using OSS gets you 98% of the way, Dropbox probably gets you 100% of the way.

5. Performance. I don't know what Dropbox is doing, but about a year ago I tested out Google Drive, SkyDrive, Dropbox, and SugarSync. I didn't want to use Dropbox because it gives you the least free storage, but Dropbox was consistently 2x faster than the competition (without using LAN sync). That makes a huge difference so I switched to it, and if Dropbox engineers are beating out Google and Microsoft engineers, they're clearly doing something special.