Let's just dig into one aspect of a file syncing app: storing the files at Dropbox scale. Without spending too long thinking about it, I'd say we need:

  * An API to request files/upload files
  * Authentication/encryption (how do we ensure DB employees aren't arbitrarily reading data on servers?)
  * Service that shards data and handles multi-region replication
  * That entails multiple datacenters (how much code does it take to keep a DC running?)
  * Automated backups to cold storage
  * Automated restoration + testing of cold storage volumes
  * Soft + hard deletion (deleting hot/warm/cold storage volumes reliably)
  * Error handling (retries, host errors, network failures, filesystem corruption, finding data when a host dies)
  * Fail-safes like blocking requests when hosts can't handle them, shedding load, etc.

I'd also add that at this scale, very few libraries start handling the types of problems you have. For example, there might be a memcache library that makes writing queries easy. However, when you have thousands of memcache servers globally you can't just hardcode IP addresses anymore. You know have to write your own custom 'service-service' that lets you look up what host you should talk to for something.