In the U.S., one difference is that they have more convenient and reliable dispatch than regular taxis, focused on virtually "hailing" a nearby car through their mobile app, rather than the two traditional options of: 1) trying to flag down a taxi on the street, or 2) booking one over the phone or internet and then waiting for it to come.
I think that's less novel in Sweden, though. Taxi apps are already common, and "virtual hailing" is how most younger people already hail taxis. In addition, there are a number of taxi companies, each of which can set their own prices and position themselves in different market niches, so you can choose one you like. In that market, Über may just be another taxi company with an app, and not have enough differentiation: http://www.thelocal.se/blogs/bostonblatte/2013/04/26/stockho...
Starting in SF probably helped them, too: SF's taxi system is worse than most.
I've wondered that myself for a while - I guess there are problems with the market for taxis in the US but that level of service has been available here for a long time.
I found it hard to figure out from their website how they differ from any other taxi service with online ordering and payment.
Also, if there are rules in Sweden about meters and price displays, there's probably a consumer-focussed reason for them.