I personally have 2 strong dislikes for the M16/M4 platform.
The first is the week ammo. I realize it works well for the military, etc, but when I don't have backup, or an automatic weapon (ie, AR-15*), I want a round that will put down to stay down.
The second strong dislike I have with the platform is its weakness in the stock. I am personally an M1 fan, so take that as a reason: but I want a weapon that works just as good as a club
I realize the M1 is too heavy, too long, has too much recoil, not enough ammo capacity ,etc for most, but its godsend reliability, its massive-hard hittin round, and its amazing range make it a great weapon for a medium-long distance military.
* the AR-15 IMHO is a worthless weapon. The main advantage of the M16/M4 is that its an easily controllable automatic weapon that anyone can use. The AR-15 is just a semi-auto .223. If I have to have a semi-auto, there are better choices.