Warspawn, COD is NOTHING like real war, think of it as an AH equivalent of a 25km/25km dogfighting arena where people fly spitfires only and spawn at 20K.
On the top of that, it depends which "game mode" u on.
1- Team deathmatch easy will provide your position to everyone. Go team deathmatch hardcore, at least. When u kill a guy, it will turn his camera angle toward u, even on hardcore.
2- Most know the maps better than you know your own kitchen. There are only few spots for sniping/ambush, u can bet that those guys already have they guns pointed at you when they turn around the corner.
3- The highest scorers run around and kill as many as they can before they die. Except some rare cases of sniping, u will not live long if u stay in one corner.
4- They are THAT good, will do a back flip and poke u in the head at 200y. Look at this guy in the vid :
http://www.youtube.com/watch?v=x55fifmC5kk, many like him online, u don't stand a chance when u still think about setting up ur little love nest, hoping u'll surprise a guy or 2.