I'm not saying that QT doesn't have great food, drinks, etc. (it does), but it is the service that sets them apart form the rest. Most decent convenience stores have items that are comparable to QT, some even better QT in certain areas. When it comes to service though, none of the pretenders can compete with QT.
The cashiers at QT will take down a line 20 deep in 3 to 4 minutes. 30 second transactions FTW!
A buddy of mine worked there during college and said that they have some sort of checklist that they are expected to perform for every customer. The company sends in secret shoppers and rewards them with a decent cash bonus if all the checklist items are met. (Think it's like 200 bucks or so, but I can't remember.) The whole greeting you when you walk in and saying something to you as you leave are both on the list. Plus, how often are those mofos spraying down the concrete around the store? Every time I go in there later in the evening, they seem to be doing it. Not sure if I have ever seen anyone at any other convenience store spraying even once.