Currently, if a replica is too stale to catch up, the status changes to 'UNK'. This is a catch-all status and does not help me see that it's still running but unable to catch up due to it being too far behind.

This status indicator should be changed so it indicates 'STA' for STALE, meaning it CANNOT catch up and needs to be killed and re-synced from the primary.

Of course, in a perfect world, MongoDB would detect this situation and one could configure a replicaset to automatically re-sync from the primary when this STALE state occurs. In the mean time, however, indicating it on the MongoStat would be a good start.