The problem of GTA is that when you play it, you kill people just like that, they die, you go for next. You do not feel that what you have done is a bad thing. Actually GTA even encurages you to kill like Killer frenzy powerup etc. I don't say I do not like GTA, I realy enjoy it, but it's just a game, you know it is and you don't care of the "people" in it. In Postal, you do feel, that they suffer, that killing them is not what you should ever do and if you play it till the end, the final twist makes you really understand that.
You got a point. The whole suffering prospect is something you don't see in many games. (if any).
(On a side note, how come my review became a forum topic, I'm somewhat new to these forums)