Here goes. In case I miss something read through all the pages of this:
The #1 Compact, Full Size, and Modified M&P Pistol Source :: MP-Pistol.com
M&P problems or changes to the pistol:
1. Mag catch - early guns had problems with mags dropping out when they weren't supposed to.
2. Slide release - design was changed to make it easier to operate.
3. Loose sights - this has been a problem for a long time.
4. White dots popping out of sights.
5. Recoil rod - they went from a rounded end to a hex end.
6. Slide finish - a LOT of guns made in 2007 had terrible finishes on the slide.
7. Magazine finish - early 9mm and 40cal guns had glossy finish which was changed to a matte black finish.
45 cal guns had black finish which was changed to a gray finish.
8. Magazine springs - all calibers have switched to stronger springs.
9. Magazine followers - 9mm has changed design twice.
40cal has changed design twice.
10. Slide stop spring - compact guns have had problems with early lock back. Newer versions have stronger springs.
11. Sear block housings have changed. A few early ones had problems with loose housings and loose ejectors.
12. The design of the striker channel liner has changed.
13. Striker breakage - this is a biggy. Lots of people have had problems with broken strikers. They are currently on their fifth design. The newest strikers are silver colored. I replaced all my strikers with the silver ones.
14. Sights were changed in 2010 to "stepped" sights. This allows for one handed slide manipulation.
15. Sights installed backwards. I've seen this on Pro models.
Enough reasons? Personally I wouldn't buy an M&P that wasn't made in 2010. I would consider the pistol good to go if it had the new style stepped sights and a silver striker. Lock the slide back and look underneath at the leg of the striker. It is easy to tell if it is silver or black.
As an absolute minimum I would get a gun with a serial number that began with "D"(2009 or 2010) and replace the striker with a silver one if it had a black one.