Caps. If Comcast institutes caps which result in people getting charged for going over, they will lose more customers to FiOS in my area - plain and simple.
Right now, the price of Comcast with promos is about $10-$20 month cheaper than the equivalent FiOS package. That is the main reason why I stay with Comcast. I would much rather pay another $10-$20 to not worry about getting hit with going over a cap, plus have way more upload speed as an added bonus.