Coverage is just one of many reasons to dislike AT&T. If you happen to live in an area where AT&T has coverage(Which isn't a lot of areas) then of course that alone wouldn't bother you. People dislike AT&T because they are by far the most greedy carrier, the most controlling(AT&T believes it has the right to say what you can and can not install on your Android phone), and you get little for your money. The iPhone is the only reason why AT&T has remained relevant over the last few years, not because of any kind of service or value they provide over their competition.
AT&T's data plans fall right in line with Verizon's and only a few years ago Verizon was working to force the phone manufacturers to use
their OS for control reasons. All of the carriers suck, but people also place unrealistic demands on them. I'm all for data tiers as long as it is priced appropriately and explained right (which none of them do).
We also expect networks which roll out quickly in Europe and Japan to do the same here, but the population density of the US is 87.3 people/mile^2 while the EU is 307 people/mile^2 and Japan is 878 people/mile^2. It is a lot easier to justify reaching every square mile of the country when there is so much extra money in each mile.
The United States population shafted itself on plans, people in the U.S. would naturally prefer to know exactly how much they're going to spend every month even if that means over paying. People in other countries are less likely to do this and prefer prepaid or by use.