In my ideal world ISPs should honor customer-provided QoS flags, while limiting total bandwidth as necessary. They can also limit high priority traffic to a certain percentage of a user's total available bandwidth (i.e. 50mbit/s connection, only 8mbit/s may be high priority when the network is heavily loaded). Anything beyond that is abusive, and will reduce the amount of bandwidth-intensive innovation coming from startups that can't afford to pay off the ISPs.