All the breathless advocates for one method or the other of releasing a slide, or for adamantly defining a slide stop/release ignore how most pistol manufacturers configure the thumb lever. The serrations are generally on the top, or biased to the top, so the thumb can push down and release the slide. One can certainly push up on the lever to stop the slide, but the lever is configured to be most effective at releasing the slide. Polishing the parts is a good idea, and so is working on the engagement angle of the slide stop and the slide notch if you desire an easy let off. It is no different than a hammer and sear where engagement angle and spring pressure determine trigger pull weight. Unfortunately, the full compressed pressure of the slide return spring works on this one engagement point when the slide is held retracted. Someone will likely market a bigger slide stop lever for M&P's just as has been done for other semi-auto pistols. I have one on a Browning Hi Power that could pass for a small kick starter for a motorcycle.