Just to clarify what MCAS really does.
I'll point out the circumstance in which MCAS should activate, because there is so much misinformation going on.
1) Airspeed is decreasing and the pilots have not noticed or they ignore the airspeed indicator.
2) Physical stableiser trim is activated (disk spinning backwards) and the plane pitches up, if this ignored then...
3) Pitch limit indicators on the digital gimbal appear. If this is ignored then...
4) An amber line is drawn on the airspeed indicator. If this is ignored then..
5) A "Buffet Alert" message is displayed on the screen (called the FMC CDU). This is to warn that the aircraft may start shaking. If this is ignored then..
6) Air speed indicator starts blinking amber and the plane calls "Airspeed low, airspeed low". If this is ignored then..
7) Air speed indicator turns solid amber (normal is green). If this is ignored..
8) The airspeed indicator displays black and orange stripes (barbers pole). The aircraft should start shaking now. If this is ignored then..
9) Motors in the stick start to shake it violently like a force feedback joystick. If this is ignored then..
10 The plane stalls.
11) The pilot must pitch forward at this point. To ignore would be certain death. Assuming the pilot is now no longer ignoring the plane he would.
12) Deactivate the autopilot if engaged.
13) Push the stick forward. If this fails..
14) Trim forward. (The assumption is this will work, or death). When the plane is pitched forward then..
15) Retract speed brake.
16) Roll wings level.
17) Start to slowly increase thrust.
18) MCAS kicks in to ensure that as recovery thrust is increased, it doesn't force the plane back into a nose-up position if too much thrust is applied.
It is simply untrue that MCAS is always active and adjusting control surfaces like you would get on an unstable fighter plane. The 737 Max is an aerodynamically stable aircraft that does not require fly-by-wire. It has hydro-mechanical physical cables between the stick and control surfaces. If the plane loses power, the pilot can still direct the control surfaces manually, including the trim. The plane is so stable it can glide without thrust approximately 10-15 miles, per mile of altitude lost. Compared to other similar sized aircraft, this glide ratio is considered high (therefore stable).
MCAS is not needed because the plane is aerodynamically unstable, it is needed because the engineers performed a "What if this goes wrong, then what if this, then this, and this and this, then this" analysis. Even when steps 1 to 10 above occur, it still doesn't start to affect the control surfaces until step 17 to help the pilot recover. It's the ABS of stall recovery, and it is this that has gone through certification.
Neither is MCAS there to mislead the pilot so he can pretend he is flying a different plane he holds certification for while MCAS changes his control inputs behind the scenes to match new flight characteristics. MCAS is not certified for constant manipulation of the control surfaces so it flies like a different plane as so many commentards here believe.
The issue is not the aerodynamic stability or instability of the plane. The issue is that faulty sensors triggered the MCAS system into activating because it could "see" that the pitch was too high and airspeed was too low.
The newspapers however see MCAS and have decided that it's existance must be because the airframe is designed faulty or unstable and this fault is being hidden from pilots. It isn't. This is like pointing to ABS and deciding that all cars that have it must be faulty. Yes a faulty ABS can cause a car to lose control during an emergency manouver and crash. In this instance a faulty sensor caused the MCAS to activate and crash the plane.
There are also some reports in the papers that the actions taken by the MCAS were too aggressive and beyond it's limits. If this is the case then like the sensors, there is clearly a fault that needs fixing, and that fault is still not the airframe.
MCAS is a system that will become standard on all aircraft, and already is on other aircraft under different names. It's supposed to be there to assist the pilot in recovery and remain inactive at all other times.
TL/DR - A safety feature mis-activated bringing a plane down. The plane itself is aerodynamically stable otherwise.