Now that the new MCAS is firmly implicated, I wonder why it was implicated on the MAX. One possible reason was that it was considered to be required by transport airworthiness certification via section 25. But looking through section 25 it is hard to find which exact requirement. The full regs can be easily googled (can't post as am probationary)
The previous requirement for STS is fairly easy to understand, as section 25.173 calls for "(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots". Effectively the certification standard demands the aircraft emulate (some!) characteristics of a theoretical longitudinally stable aircraft which would have:
a) COG significantly ahead of the Neutral point
b) generally have the trimmed horiz stab flying at significantly lower AoA than the wing (probably to the point of downward lift at the tail)
However for modern air transports, which are desired to have optimal efficiency, it should be best to operate at near neutral longitudinal stability (i.e. just stable). In this configuration the tail is generating very little lift in either direction and hence very little drag. "Manual flight" is really "Simulated longitudinally stable manual flight necessary to conform to 25.173", which is confusing because as other posters have noted, this is really a less-automated mode instead of a manual mode. It's not so relevant to this thread anymore, but the STS seems more regulatory than useful really...changing the stab trim position via STS does not really create a more longitudinally stable aircraft and the pilot is likely to respond by removing the trim input anyway to avoid unwanted altitude excursions. This leaves him back with a slightly stable aircraft which would probably cause fatigue if pilots were expected to fly whole sectors in "manual mode", but of course they are not.
So why MCAS? It is not a speed stability system, it appears to be a beefed up stall prevention system. If the MAX COG has crept further back, it's stability margin has decreased but I don't see why an antistall stab trimmer becomes a good idea or a regulatory requirement. Looking through the stall/stall warning sections (25.201, 25.203, 25.207) there is a preference for aggressive pitch down at stall which may have something to do with it:The airplane is considered stalled when the behavior of the airplane gives the pilot a clear and distinctive indication of an acceptable nature that the airplane is stalled. Acceptable indications of a stall, occurring either individually or in combination, are—
(1) A nose-down pitch that cannot be readily arrested;
(2) Buffeting, of a magnitude and severity that is a strong and effective deterrent to further speed reduction; or
(3) The pitch control reaches the aft stop and no further increase in pitch attitude occurs when the control is held full aft for a short time before recovery is initiated. If anyone has a better understanding of how MCAS is dictated by section 25 airworthiness would be very interesting to hear.