I was under the impression that the M&P series were suppose to be DAO pistols. After examining my new M&P 40c to see how they pull off such a light trigger pull, it appears to be Single Action.
First, let me explain my understanding of DA and SA. A trigger pull in Double Action both cocks and releases a firing pin, while a trigger pull in a Single Action just releases the firing pin.
Examining my M&P, I see that racking the slide (to put one in the pipe), pulls the spring loaded firing pin back. It remains in this position until you pull the trigger. Pulling the trigger simply releases the firing pin/striker from it's armed position.
As most M&Ps are sold without safety (they do have versions with safety), and the Compact series are considered personal defense/carry weapons by many, I would say that there's an increased risk in carry over what many considered SA Semi-Automatics before (mostly 1911 style).
We've all seen articles and posts by people who consider 1911s unsafe for carry. While I personally believe they are very safe considering a 1911 style gun has three safeties. These are the actual thumb safety, the grip safety and the half cock safety designed to engage if the hammer falls without a trigger pull.
Maybe I'm missing something regarding the safe carry of the M&P series as most of my handgun experience has been around 1911 guns, true DA semis and revolvers. But I can't understand how a M&P series without a thumb safety is considered a very safe carry weapon. The firing pin/striker is "cocked" and the only thing between it falling is a trigger pull.