Myra Kraft. The Patriots don't talk about it (too much) but in addition to the normal motivation for every NFL player to win a SB, she's providing that extra motivation. I know its sappy and all but anyone who's played a competitive sport knows how you look deep down for that something extra when you near the finish line.
Regarding players, I can see BJGE as the X factor, but I'd lump Woodhead and maybe Ridley in there too. When the Patriots go to their hurry up they still shuttle their RBs in and out quickly so that they're fresh for those quick hitters between the tackles. So my X-factor is the NE running game as a whole.