Well a few others will chime in, with more detailed answers, but I'll give it a swing.
MIM stands for Metal Injected Molding, basically is like making a casting but you can reuse the mold over and over and over. Very cheap way to make parts of metal. Problem is that gun owners are seeing a lot of problems with the Quality Control from the vendors that the gun makers are using. Notably Kimber and Springfield Armory. With good QC these parts will work just fine in some guns.
What some owners are having problems with is that the QC on these parts is falling below par and parts are breaking. For those who CCW this is not acceptable. For a game gun, range only, and plinking guns, fine replace as needed. For some of us the first thing we do is replace a new guns MIM small parts with quality replacements from good vendors. Others will shake the dice and let em fly.
So if this is a Carry gun then you might consider replacing some of the MIM parts on your pistol. "I" suggest the Sear, Hammer, Disconnect, Extractor, and Slide Stop. The problem is the fitting of these parts to your gun. As I was once told "if you can build a Model airplane well then you can put together a 1911." Most parts come over sized and rarely just drop in so a little fitting with files and poilsh paper. Some do but cost a little more (read "hardcore parts from Ed Brown.") So armed with that info, go out and ask more questions.
But that's my two bits
