This happened really early in the game and got trumped over and over again by other absurdly spectacular plays, but there was major importance here, too.
On the first drive of the game, the Saints were about to go into the endzone. On a third down from the San Francisco seven-yard-line, however, Saints running back Pierre Thomas took a short pass from Drew Brees, and safety Donte Whitner was there to lay the wood.
By any definition you want to use, that was a game changer.
If Whitney doesn't make that play, the Saints take a 7-0 lead or a 3-0 lead, instead of going into a 17-point hole. That's very likely to change the entire course of the game, isn't it?
As for the hit itself, it was clearly helmet-to-helmet contact, but by rule, Pierre Thomas was not a defenseless player, so there was no penalty. He'd made the reception and turned upfield, which means it's totally fine for a player to blast his head with his own head.
No one said it was a perfect rule, but it is the rule. Thomas did not return to the game.