The obvious problem is that if an ISP wanted it could simply throttle Netflix, Hulu, and Amazon until they die out (which would be convenient for many TV/Telecom companies).
And maybe less obvious is the start-up that Netflix once was has a tougher go of it because Comcast wants their content streamed nicely and has to keep the Netflix customers from yelling too loud and have to squeeze someone's bandwidth to make profit goals which is perfectly legal as long as they are transparent about it.
