Currently we have a few authentication startup checks in 3.0:
- check for indexes on system.users
- check for schema version
These checks prevent mongod from starting even when auth is off, but without this improvement you can sync a 3.0 node from 2.6 node with the 2.4 auth schema, and it will work just fine until you try to restart the node.
We should validate during initial sync that the sync source has a new enough auth schema version.