What is the difference between Pantheon DROPS and vanilla Drupal? Is it just the added config wrappers, or is the Drupal site code itself changed? Are there any changes to Drupal module and filesystem compatibility compared to a more traditional Drupal hosting environment?
Pantheon's variant of Drupal is openly hosted on GitHub:

https://github.com/pantheon-systems/drops-7

The main variation that matters is in drupal_settings_initialize():

https://github.com/pantheon-systems/drops-7/blob/master/includes/bootstrap.inc#L720

This is how Pantheon runs sites without making users handle DB or Redis credentials. This change is now in Pressflow as well.

The other material addition is an apachesolr integration package and a way for the site to query the Pantheon API. Those are in modules/pantheon.

The good news is none of this will prevent a site from working anywhere else. The changes are all designed to revert to normal when not on Pantheon.