Well, my concerns are the bullets don't go where I want them ... but then again, I'm new to shooting and have a lot to learn!
Honestly, as a newbie, the only thing I can say is that I find the point on the trigger just before it goes boom is a little tough to get by.
Granted, as I said, I'm new and probably need to learn proper technique ... For the moment I follow what I was taught as a child shooting my .22 rifle, and that was to keep slowly squeezing until it fired, never certain of exactly when it would occur.
The only thing I base my preferred trigger "feel" on is the LEM trigger I felt on the H&K at a gun show. It was the smoothest of all the triggers I pulled, and the point when the gun went 'click' was very easy to cross.
The H&K didn't fit my hand well enough though, not to mention a very steep price tag. I imagine the M&P can have the trigger adjusted to approach the LEM trigger on the H&K by folks like the Amazing Dan Burwell, but I want to first learn how to shoot the stock pistol well enough before making any changes.
Aside from a few broken strikers and site/aiming issues, you'll be hard-pressed to find anything truly "bad" about the M&P in any caliber. It's really too new for there to be any long term use problems, and for the moment, just about everyone here has nothing but praise for it.
For an under $500 pistol, (for some $450 in your hands), you really can't go wrong!
But hey, I'm just a newbie, so take my opinion for what it's worth!