1) The big difference with Pierce on Kobe was that Kobe couldn't post up. With his dribble penetration denied all series long, the only way that Kobe could get inside and possibly break down the defense was if he posted up. With Pierce on him and no driving lanes, he became purely a jump shooter which is much easier to shut down.
2) Many of KG's games are like that for some reason, where when you step back and look at it soberly he played a lot better than you realized. I think it's a stylistic thing, that because he doesn't play with the style that many would like it is easy to overlook just how effective he really is. In that game, though, even in live action it was clear to me that he was the lynchpin for that game. In 1st quarter when KG couldn't buy a shot, the game was still reasonable until he left with the second foul. That's when the wheels fell off and the team went down by 24. In the second quarter when KG came back in the Celts cut the lead back down to 11 - 13, then when Doc pulled him out in the last minute the Lakers surged back up by 18 right before the half. Then, in the 3rd, he came back in and again they started putting pressure on the Lakers and eventually overcame them. The fact that KG could defend the middle and just about outrebound the entire Lakers team by himself was a key reason why the Celts were able to play the small unit with Posey at the PF during the pivotal 3rd quarter.
3) House and Posey both were just nails that game. In fact, that is one of the few games that were such a team effort that you can make a legitimate argument for any of 5 players as the player of the game.