Since I've been on AT&T, I haven't had nearly as much to complain about their service as many do. My biggest complaint is that while at work, most of my building (which is substantially huge) has 3G coverage, my office only has EDGE.
The things that make me hate AT&T are their shortsighted way of viewing their customers. It bothers me that every ISP (note to AT&T Wireless: this is what you are now; even if you didn't have the foresight to see it coming, it's pretty much the case) is focusing on restricting use rather than realizing that the market is very explicitly telling them that they have a winning product, but we want MORE! They complain that their customers are using too much "unlimited", when the solution is plain:
I understand that this costs money. It requires planning, hardware, and support / maintenance. It's not easy, but it is plain to see. But the goal of an ISP shouldn't be to be the 'least worst' or 'least despised'. Investing windfall profits in a meaningful way could go a long way to making your customers actually happy. Let's face it, hating on Wireless providers and ISPs is becoming something of a national pastime. In much of the developed world, the best Internet service provided in the US isn't even considered to be broadband...
Here's a post by Fake Steve Jobs that amusingly satirizes a conversation between (fake) execs:
I think FSJ puts it well...