Willis didn't recognize blitz and adjust his route. Last year I read an article about what Harbaugh did in SF to get Alex Smith from being an Int machine to throwing very few Ints (Giants game notwithstanding). The gist of it was that they built a hot route into every pass play. Which means that if there's a blitz, both the QB and the receiver don't have to make adjustments. QB recognizes blitz - throws the hot route immediately. WRs don't have to 1) recognize a blitz and 2) adjust based on that. It cut down on Smith's mistakes when he got blitzed. Now, this doesn't solve all problems, but I'm amazed that more teams don't employ a system like that.
Now, off to see if I can find the article quickly.
Edit: Found it a lot quicker than I thought - "Quarterbacking Made Simple