Halo 2 had the largest skill gap of them all, playing online in halo 2 PC for the first time in years has shown me that in spades. The only thing that's easier then halo 3 is the sniper rifle. The halo 2 sniper rifle is a side swiping machine.
I personally enjoyed halo 2 more then any other title.
I'll give you a good example, I had a battle rifle from halo 2 in coagulation (blood gulch) and got into a range that was ideal range for the DMR from halo 4, I couldn't even HIT the guy. Cross-hairs right center mass on him. I was so used to using rifles from that range that it was a joke. The DMR has retarded range, too much range.
It's not that the BR has too little range in halo 4, because it's actually very close the range of the halo 2 BR, but the DMR just can shoot SO MUCH farther accurately that its no contest, and makes the whole game stupid.
In halo 2 if you were REALLY far away from someone you didn't have much to worry about so long as you strafed and kept moving into cover. I would get slapped if I said the same thing for halo 4.