(Sorry for that little advertisement of that screenshot up there, Ill go back ontopic)
Plus Firia, AO doesnt have some Gimp running around with a Massive Rifle killing people through walls.... wait it pretty much does.
CS is defenatly not the most realistic FPS, ghost recon and OPF beat it there, but it by far the most realistic while retaining fun FPS there is. (Flashpoint is often very aggrivating, when seeing some yahoo in a tank and your stuck with a Rifle, thats not that much fun, ditto Ghost Recon)
As Meta-lynx said about CS's realsim not mainly about one thing, but the atmosphere (Terrorists, guns, hostages, not a Hell Level), and traits of the game like shootable enviroments.
CS sometimes on one server seems like its quake III with HK Mp5s and Steryl Augs, but thats often just the server. It could have lo grav, less recoil settings, or just have players that play it like quake III. But if you find one of the many servers with good players that play it like a realistic sim, then you'll notice how realistic it is.
Real guns and Settings (As Metalynx said), real objectives, real teamwork (Snipers covering the assault teams back, Point-man and so on), and Real tactics.
Unless Im screwing around and running down a hall witha Shotgun, I play it like a close combat Ghost Recon or Flashpoint. Never stop without cover, never reload without cover, and move from cover to cover, just like the real military often does.
Would you rather play CS if it was realism filled to the brim? No jumping, no moving while firing, no crosshairs at all, and when you die, thats it, no playing that server anymore, because thats 100 % realsim, you die... your dead punk.
Try Ghost Recon or Raven Sheild when it comes out if you want more realism, CS is realistic and good enough for most of us. Again sorry for posting the flashpoint Pictures, but if Mummu or anyone else wants too much realism, just try that.