So many cool little touches in this game you can run into just doing random things.
When it's raining, if you point the camera up towards the sky, droplets fall down towards the screen and hit the camera as if it's an actual physical object in the game world. It looks quite nice. Not sure if it was the same in GTAIV.
Also, there are some pretty neat animations Martson does at times. For example, if you hogtie someone, duck over top them, and then do a knife attack, Martson will go into an animation where he slits their throat. Crazy. I was feeling particularly devious when I did that on someone, not really knowing what to expect, and couldn't believe it went into an actual animation. :lol Got a nice -100 honor for it. Not too honorable, apparently, killing someone who's hogtied and on their stomach.
There's also an animation if a person is on their knees, and you get behind them and shoot them in the back of the head.
Those are the only two I've found so far while playing. I'm guessing there are probably others.