Former USMC 0351 (assault-man), spent one tour in Al Anbar in 2004. I have mixed feelings on the M16 (carried/issued an M16A4). The last rifle I was issued was an FN contract M16A4, I had no reliability issues with it but in fairness I was systematic about frequent cleaning. I have had issues with the weapon system jamming when very dirty in training environments, so I made sure to clean the rifle like a nut every chance I got. Even if it was just breaking it down shotgun style and brushing off the bolt/carrier group and the inside of the upper. Quick down and dirty and takes maybe 45 seconds.
I don't hold it against the rifle that it wont ingest huge amounts of mud or sand and not run right, not many rifles will and those that do are not exactly known for useful accuracy. I do think that a rifle more tolerant of crud and grime would be better, I would still clean it as mentioned when time allowed but higher tolerance for muck/dust would be nice.
Although I did not have an issue with this (I had optics mounted) some Marines had to dummy cord their rear sights onto the flat tops as the securing knobs would get banged around and come loose. There has to be a better more secure way to mount the rear sight/carry handle than those stupid ass knobs that stick out 3/4" to catch on everything. Same for some of the optics mounts, for example the mount for the ACOG has the same issue. I think they would stay tight if they were not sticking out so far that they caught on everything.
Then of course we get to talk about magazines, which although not the rifle itself are still part of the system. I will come right out and say that M16 magazines are too delicate for hard infantry use. Most feeding problems are magazine related and M16 magazines suck.
My other major gripe is with the 5.56mm round and it's poor performance. Not everyone will agree with me, but the 5.56mm ball round is not exactly a DRT (dead right there) kind of bullet, and it is also not terribly effective on mud brick, engine blocks, or things like that. I am not saying we should all start humping around 7.62NATO (although every squad should have an M240 and plenty of ammo) but we need something in between that has more power. Unfortunately a round that would meet the needs won't fit in the above mentioned crappy M16 magazines or M16 magazine well.