The M&P is an attempt by S&W to get back into the police/law enforcement market, and perhaps the military market at some point.
The ability to field-strip a striker-fired pistol without having to pull the trigger in order to decock it is likely to become of interest to at least LE agencies.
The system developed by Walther for their 99 series (present in both the Anti-Stress and Quick Action models), and shared in the traditional double action versions of the SW99, seemed to attract an unexpected amount of negative feedback from the market place.
The elegantly simple system designed by S&W engineers does add an extra layer of user-involved safety, which is something rather marketable when it comes to LE sales. The sear deactivation lever's shape was changed, essentially for ease of reassembly if a user was a bit 'inattentive' regarding its position when installing the slide onto the frame after cleaning. The original one has a dog-leg curve and the current one is straight at the end.
Yes, the design used by Ruger in their P-series is easier to access with just a fingertip, but then that's likely because it incorporates the ejector as part of the plate, and the ejector is what is most easily used to move the plate into the take-down position.
The M&P magazine safety is also a surprisingly simple design. Somebody had their thinking cap on when they designed it.
I've carried S&W service pistols which have been equipped with magazine safeties since 1990 and it's never bothered me. The magazine safety in the M&P is an even simpler design and it wouldn't bother me in the least.
Of course, as with anything else in the marketplace, everyone is certainly entitled to their own opinions ...