Already answered in that thread. New CPU, and new power amplifiers for the wifi.
Does the RT-AC1900P have the same FCC ID as the RC-AC68U series routers?
Does the RT-AC1900P support higher power (1 watt) on the lower 5GHz channels, or are all the AC 1900 series routers limited in that respect to pre-2014 FCC indoor restrictions?
Do any popular routers support DFS channels in the U.S.? At full power?