I suspect how it will work out if it happens in the US is rather more like how Telecom NZ or Vodafone operate (Telecom used to be mainly owned by SBC/AT&T).
Where either you don't have the option of a data block past a certain upper bound, and get shaped to 64kbits for the rest of the month (Vodafone).
Or you have the option to remain full speed but the overage charge is out-of-this-world insane, something like 2c/MB. That's right, per MB. 10GB over cap is $200 added to your bill. Ouch.
If the telcos are not price gouging, there shouldn't be a problem with data caps. Stop being a cheapskate!
>Even if it was possible to implement QoS to broadband
It most certainly is possible.
>Why should someone in a busy area be capped when they reach the limit
What limit? I'm arguing that there shouldn't be a limit. You just provide a guaranteed minimum bandwidth. When the line is under-utilized the connected clients can use as much as they can. When utilization goes up traffic is shaped down as far as needed to handle it.
With data caps then people in your less busy areas will find themselves completely offline toward the end of the month while the line sits idle. Why?