In AA you can shoot thru doors and some walls too.. but it's a lot more intelligent. The sniping is a lot harder because it makes you breathe in and out, and you the SOUND in that game is incredible. I'm sorry, but CS:Source is rather laughable compared to AA... and I played all the other shooters, AA is my favorite because it takes real skill to play. This night I played, I noticed I did awesome when I figured out the rush patterns and just camped a few spots. Then I go to dust and I am getting headshotted from pistols across the screen -- it's unbelieveable. I will continue to learn to play CS but it pales in comparison to AA for the squad based combat, which takes a combintion of quick fingers, good listening skills (the sound rawks!), common sense for covering objectives and a bit of tactics thrown into the mix.
AA reigns supreme of all the shooters out there right now. Quake3 and UT2004 are more fun if you want the twitch action FPS, and CS:Source/CS well.... it's rather unremarkable as it's nearly identical to the game with the crappy graphics. I am glad that HL2 itself seems it will rule, because CS:Source isn't worth the extra dough. I hear things about DOD... maybe that will be a bit better and not as lame with the pistol headshots from across the screen.