For me the real kicker was that Boeing convinced the FAA that the aircraft did not need redundant Angle of Attack sensors, even though they knew that this meant that there was a single point of failure in a system which had control authority (ie, MCAS).

Boeing did not disclose the existence of MCAS to pilots, and therefore did not train the pilots on how to recover from MCAS failure, and there was no redundancy in the key sensors that fed MCAS.

IIRC at least one of the crashes was caused by an AoA sensor failure.