What really is needed in the US with all phones is more universal phones so we don't have to chase down which model the carrier has available. Right now Verizon has 2 nice small WPs and no big one at all. ATT has one big one and nothing else. With other smaller carriers the situation is about the same.
Of course there are many WPs that can be bought and used on more than one carrier, but you have to dig in the specs to figure out which ones because even though GSM is more widely dispersed among carriers, they all want to play their own games with it. That is one very big plus with iOS phones these days, all current models can be moved from one carrier to the next without much effort.
IMO, it is a shame that MS doesn't have the clout these days to just tell the carriers here's the phone, it will work with all.