Functionality of the App and website and also customer service. Several complex itineraries cant be booked online and once you book making other than simple changes on the website is difficult or impossible. And they dont even have 24h customer phone service. Few months ago I had a flight leaving US about 11.30 pm on Saturday and then at 8 pm I found out trip needed to be cancelled. There was no means of getting in touch with the airline. The phone line said they are closed for the day. Tried calling number in europe and not 24h either. Only option of speaking to airline representative at 8 pm ET was to actually go to the airport. And some of their 'non-refundable' fares are just that...forget about flight credit. I would chose United over SAS every single time because I want to be able to make changes or speak to a human being anytime I need.