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The profile of the barrel is altered, first of all by making it thicker, the main idea being that it would heat up slower from firing, though this doesn't seem to have made much difference, the second is that there's a step in the barrel, which is for mounting an M203 grenade launcher.
Likely one of the additional intents was that it would make mounting the launcher more secure, but I've never heard of it being a problem on the thinner barrels of earlier rifles and carbines.
Other rationales includes that it would improve accuracy (but it doesn't seem the A2 barrel profile actually does), one that I've been told is the main reason is the simple fact that some soldiers had been known to sometimes use the barrels of their M16A1 rifles as pry-bars, which had in some cases lead to the barrel bending. It seems this could be easily solved by just slapping them in the back of the head and making them do pushups, but to some official somewhere this stupidity needed to be addressed with design changes.
An addition I actually like a lot is that there's a wedge behind the ejection port, this is a brass deflector, ejected (potentially pretty hot) casings would bounce against it and not come directly back against either you if you're left handed or have to fire it left handed, or possibly at the guy standing right next to you.
The thicker barrel profile, and the additional material on the receiver bodies (brass deflector and reinforced buffer housing), together make the rifle weigh a good bit more, while the M16A1 was quite lightweight and handy at 7.9lbs, the M16A2 is almost a pound heavier at 8.8lbs, which matters.
