MCAS to me is not a "system" per se, but simply an additional subroutine in the Flight Control Computer. Albeit, a poorly coded one. Most computers attached to flight controls have multiple channels that compare their inputs before making an output. A single faulty AoA sensor activated the MCAS subroutine of the FCC. Flaps up, high AoA (stalling), MCAS commands the existing trim system to trim the nose down until the high AoA indication goes away. A single failure should not have activated this logic. That's why commercial aircraft have at least 2 of everything.
This being said, anyone who has flown a 737 classic / NG / Max knows that commanded or uncommanded trim wheel movement is very noticeable - both thru sight and sound. The crew the day before the mishap flight readily identified the uncommanded movement and completed the QRH. The mishap crew did not. As to why they didn't, hopefully we will find out.
Why is there MCAS software on the Max, maybe the FAA was the driving force. It's interesting that this year the FAA has put high emphasis on full (not stick shaker) stall recovery during AQP. Maybe this is yet another result of the Colgan Air 3407 crash in Buffalo, NY, 2010?