Safeties.
First thing I do when I get home is sit down, pull the decockers/spring out of the frame and install the safeties.
First handgun I carried and shot a lot was a 1911. Thumb safety. I've had other guns without thumb safeties (Browning BDM, Glock, XDM) and moved on from them. Never got them to shoot like the old 1911s and kept looking for something that would shoot similar groups.
The M&Ps, FNS, and one XD have thumb safeties but the majority of them won't shoot to my standards.
Why? I normally carry a P07 with thumb safeties. If I were to find another gun that shoots to suit me (the M&P Shield 9MM does and it has a thumb safety, too) that I might want to carry it should operate the same way.
Consistency is very important. People have proven that under stress you react as you've trained to react. So what do you do when you carry a revolver one day, a semi auto with no safety another day, a semi auto with decockers another day, etc., etc., etc?
Whether I'm carrying the usual P07, the P01 Omega, the M&P Shield, or (very seldom) an FNS-C .40 that also has thumb safeties they all operate the same. Same way to carry it, same way to make it go bang, same way to drop and empty magazine, same way to load a fresh magazine.