I'm NOT a pro player or a sniper guy, just to make that clear. Im a regular average player who uses all guns and snipes on occasion. IM here to give a rational non bias breakdown of QS. Ok lets start now thats out of the way.
The major problem is that cod is a pier connection system. One person is host and everyone connects to that chosen players xbox. Im linking this to driftOrs vid to better explain . http://www.youtube.com/watch?v=xyCQtUFOJmA
Now the point to take from lag is while on your screen you've put 6 shots in sometimes in your match to the snipers screen you didnt even have your gun up and he'll one shot you. Who wins just depend on your xbox's connection to the host's xbox
Now to clear up the how snipers kill you without looking down their sights. This is simple solution - its a killcam/theatre glitch that doesnt show the sniper looking down their scope. When in fact they ADS and shot you.
Next is the people who claim snipers can get one shots to the legs. They cant. the reason this appears to happen is because the hitmarker and his crosshairs hit you leg. what really happened is the sniper ADS to fast and the bullet flew off from sway, hip fire, or scope delay. While treyarch patched ADS delay snipers still a slight delay in the scope where it will be up but not accurate(scope delay).sway explains itself.
Im going to link a video by 402thunder402 to better explain my point. In the clips thunder gets killed by a sniper; pre ADS delay patch. So the snipers bullet flew off course by a mix of these methods or just ADS delay accuracy Snipers can still fire to fast before the scope is up. - http://www.youtube.com/watch?v=vV1517OZnSo
No about the hit markers on the legs. Once again a simple misconception hit markers will ALWAYS hit the center of your screen.
I hope I've made you understand quickscoping a little more so you dont get as frustrated and can explain to other people so they to will understand