Whoever shoots first with an assault rifle doesn't win. Some people are better at aiming than others(particularly noticeable over long range), and there are also factors such as drop shotting, and whose gun is better suited to the range, which make a difference.
Quickscoping took advantage of auto-aim and snipers being so powerful to be brilliant at any range. For those who say "But SMGs can be used at any range", go and play one of the games before they took out idle sway. Yes, you can kill someone from the other side of the map with a SMG on Cod4. You will need to have some skill though, and you will need to adjust for idle sway and recoil over the long range, and fire in very short bursts. If your opponent just runs, they have a very big chance of survival. If they have a gun better suuited to the range, they will kill you most of the time.
This is sadly less true in the games after MW2. In MW2 it's because of lack of sway and recoil.
In black ops, it's better, but because there isn't much sway, it isn't perfect. ARs can be beaten by SMGs from longer ranges than they probably should a lot of the time. Snipers can be beaten by ARs from longer ranges than they should. This is, imo, down to a lack of sway in part, but more because of the map design. There aren't many places that snipers can snipe from where they can't be seen by someone with an AR.