I've been a Glock owner for about 9 years or so, and had shot a few M&P pistols since they came out. Now, having just acquired my first M&P I thought I'd take a few minutes to share some of the design similarities and differences between the M&P and the Glock pistols.
The M&P that I picked up is a full size .40 and is comparable in size to the Glock 17 or 22. It is a bit larger than the 19 or 23 size Glock offerings. I've got the M&P posed with a Glock 19 and 17 here for comparison. First similarities that jump to mind is that both makes are polymer framed, and striker fired pistols.
The 17 of mine has had a few customizations along the way. I have cut the grip down to accommodate the shorter G19s 15 round mags in addition to any of the larger capacity 9mm Glock mags. Most of the comparisons will be of the M&P alongside of the 17 since they are the closest in size. (pardon my dirty G17 in the pictures as it is kind of a high mileage beast and is kind of my range mule. It doesn't get cleaned after every outing, but usually once a month or so.) One thing to notice is slide length and sight radius. Very similar dimensions in both regards here. The rear beavertail on the M&P makes it just a hair longer overall.
Taking the pistols apart we can compare the frames a bit to see the fire control parts and compare some of the internals. (M&P on top, Glock on the bottom) First we can see the differences in take down levers. The M&P uses a flip down lever that is rotated downward with the slide locked to the rear. The M& P has a yellow tab inside the frame that can be pushed to decock the striker before the slide is moved forward. Or the trigger can be pulled like the Glock. The Glock uses small pulldown levers on both sides of eh frame that are pulled down with the slide retracted slightly to the rear after the trigger is pulled on an empty chamber.
Other things to note on the frames are that the triggers are different in execution of trigger block safety. The Glock uses a split trigger with a lever that must be pushed to allow the trigger to move rearward. The S&W uses a hinged two piece trigger that requires that the lower half be pulled to move the block out of the way and allow for rearward travel. Also the Glock trigger guard is kind of beefy and squared at the front, and has a deeper undercut near the point that it joins the frame near the grip, allowing for the gun to sit a bit deeper in the hand. Again this Glock has a extended mag release, and extended slide release as modifications, but both guns exhibit well placed controls that are easy to get to. The other big thing the M&P has is the interchangeable back straps for fitting different hands. Current Gen 4 Glocks have this as well, but my old Gen 2 Glock does not.
When we look at the fire control parts inside the two pistols, (Glock on the far side and M&P nearest), we can see that the trigger bars are similar in design but have located the portions that move the striker block in different locations. The protrusion that accomplishes this for the Glock is further forward and has a shallow and longer angle that goes at a constant slope. The M&P has a multi- angled protrusion that has a steeper slope. This difference is perceived during the trigger pull. In the Glock it feels like a constant build of pressure as the trigger bar moves rearward. In the M&P it is perceived as a three part series of small gritty clicks near the end of the trigger pull. Also notable are differences in sear blocks. The Glock unit fits inside the frame and inside the slide rails, which are stamped metal inserts and is made out of polymer with metal parts inserted. The M&P Uses a sear block that is cast metal and houses the sear parts an also has the slide rails cast as a part of this block. The ejector is part of the block on the Glock, and on the M&P it is a pined in part along with the sear and the sear spring and plunger. The sear is a separate small MIM part on the M&P while on the Glock it is a stamped part. Total individual trigger parts are fewer on the Glock which makes it simpler. Whether that is better or not is a matter of opinion.
Looking at the inside of the slides we can see other minor differences in execution but design concepts are about the same. Again both slides are about the same in overall length, the M&P slide is just a bit thicker at it's wides point. the biggest differences are in the placement of the striker block, safety. That safety is seen as a silver metallic circular plunger on the underside of the slide. This is what that protrusion on the trigger bar must push up and into the slide as the trigger is pulled rearward to allow the striker to move forward and strike the primer. This prevents the striker from moving forward unless the trigger is fully depressed to the rear. This can prevent discharge when dropped or with heavy impact. Of course with the trigger not pulled the striker is also not being pulled to full cock either, and is only partially set.
One can also see that both pistols use a flat wound recoil spring. The M&P uses a Steel guide rod while the Gen 2 Glock uses a plastic one. The barrel of the Glock uses polygonal rifling, which is easier to clean, and contributes to higher velocities slightly. The down side is that it is not tolerant of lead bullets, and hand-loaders often will replace the barrel or skip Glock all together. The M&P barrel uses conventional rifling, and will readily use lead cast bullets. Slide construction is similar, but Glock uses a tennifer treatment to their slides to inhibit corrosion under their coating. The S&W uses a Stainless steel slide and protective coating.
As can be seen the M&P slide, (with the green on the recoil spring or on the left in the bottom picture), has the striker block plunger further to the rear. Both the Glock and M&P plunger have compound angles, or sloped sides to make the upward push into the slide go smother. However, the Glock plunger sits a bit deeper and does not have as much vertical surface exposed beyond the surface of the slide, while the M&P has more of the vertical surface exposed. If we go back to the pictures of the slide, and look at the trigger bar protrusions that engage these plungers, you can see that the multiple angles on the M&P and steeper angle help contribute to the crunchy feel of the end of the trigger pull. Having a radiused plunger may help this smooth out.