Yeah, I noticed the lack of action taken by the NPC in question too. I think it's unfair to judge MGS3 by what we've seen so far though.
I remember being quite impressed with the AI in MGS and MGS2. They didn't make things too easy, but they still made it so that you could perform some pretty cool manuveurs.
Watching the MGS3 vid was a lot like watching an action movie's trailer, and speaking as a person who's watched A LOT of action movies the "bad guys" aren't always the smartest.
Snake is meant to perform some pretty inhuman feats (like shooting 5 guys standing next to each other and having none of them notice until it's too late) and that's what makes the game great.
I think you should ask yourself. Just how realisitic do you want them to be? So realistic so that every little thing you do, they hear, and call you on it? So realistic that it'll take you a good hour or so just to sneak up behind and kill one guard?
Also, keep in mind that it is a game with varying levels of difficulty, and that guards in the beginning may not be as smart as guards closer to the end.
Just some stuff to consider I guess. On the other hand, I'll be the first one to complain if I take a shot at a guard, miss him, and have him stand there with a stupid ass question mark over his head.