This isn't a genre at all, but just a particular sound.
Thrash Rock, in my eyes, is the true mixing of punk rock, hard rock, and classic metal. It's not punk outright, but it's not really heavy enough to be called metal. It's not like a traditional uptempo song, either.
The best example I can give are these

http://www.youtube.com/watch?v=VOMoHXEhblo
http://www.youtube.com/watch?v=yyno4Ugo8t4
http://www.youtube.com/watch?v=cRXDoQiirPI
http://www.youtube.com/watch?v=CaCGS46qFnc

(I'm sure Deep Purple has a few, though...)

Like punk, it's uptempo, perhaps just a tad bit simple. But unlike punk, there are riffs and solos more akin to melodic forms of metal. It's what I thought '70s speed metal would be. Imagine what you would have if you crossed Black Sabbath with the Ramones. The riffs and solos, with that unique brand of punk.

Definitely faster, but it doesn't rely on guitar crunching or double bass pedals. There are few- or even no- 'slowing down', or 'quieting down' points in the whole track. In essence, it's loud the whole way through, not loud-quiet-loud. It feels like every part of the song flows the whole way through with no real gaps.
It's just hard to describe what I'm saying.
Does anyone know songs, bands- or what the hell I'm talking about- that might be similar?

I'm moving this to the rec central, cool down on the threads, dude. I told you before.

Sorry.

By the way, add MC5 to this mix. Kick Out The Jams is definitely within the boundaries of what I'd consider 'thrash rock.'