For me, the most useful tech talks are the result of a lot work by the speaker (or the speaker's team). My favorite example is Charlie Miller's talk on battery firmware hacking at DEFCON 19[1]. In 45 minutes, he detailed months of time spent trying to get a laptop to explode remotely. He gave examples firmware disassembly, research (he'd never done hardware stuff before), and social engineering. Not only did the talk contain information about battery security, but it gave a glimpse of the thought processes and tools he used.
Compare that to conferences about politics and philosophy. There's no equivalent talk. They use debate formats because what else could they do? They don't have much useful information to present.
1: http://www.youtube.com/watch?v=dlSBQ5b6Pdw#t=0m35s