There is more to serving to than just raw power. Federer still probably has one of the best serves in the game. He uses it not only to hit clean winners (aces) but to set himself up as a first strike weapon to win points. At the end of the day it is not about the number of aces you hit but the points you win on your serve. Though in saying all this... he certainly did not have a stellar day in the US Open finals.
But you do have to admire without his best stuff he was still able to push it to the limit.
I can name 5 players every year from 2003-2009 who have better serves than Federer.