SO is built with MVC, which encourages good development practices. T-Mobile looks to be using Web Forms. I'm sure there are good sites out there that use Web Forms, although as a .NET developer, I've found a that Web Forms encourages practices that I consider bad.
I agree. But I hate the crappy abstraction that is Web Forms. It throws so much garbage on top of the HTTP request-response cycle, and so many developers are content to remain ignorant of what's going on underneath or how the Web actually works.