Do you get hitmarkers but still die? Seem to die in 2 bullets or less? Then that's lag compensation.
Do you aim and you're sure (mean really sure) as hell you're on target but don't hit anything? Hit detection.
As far as overpowered goes, I haven't seen anything yet that is truly overpowered. I've taken to grabbing an AR and flanking people so I can shoot them in the back. Can't win face to face anymore.
Before the patch I was going 44 and 8 in games. Now (if I play the same way) I go 9 and 20. Overnight change. First few games on the patch was fine, then when I loaded it the next day it had all gone to hell. I think they updated the server whilst I was offline and that was what caused it.
"My name is Spartan Jay, the greatest MW3 player of all time. Because I am on your team, this team will win" ~ Spartan Jay