The B12 (XII) Forum, home of the 'Front Porch, y'all' at College Football Fan Site, CFB51!!!

As a litmus test, can you grab a skateboard and do a kickflip to the music? That's thrash.
Anthrax's drummer Charlie Benante (now married to Carla Harvey, lead woman with The Lords of Acid) is credited with popularizing the "blast beat". That's another indicator of "thrash".

In reality, thrash bands have to be careful to keep it musical. Too many just try and see how fast they can go. It stops being musical and just sounds like 4 guys soloing over each other. I've seen guys put cams on their double pedals and then play two sets, trying to get their kick drums up to like 240 bpm. There's no room for any other instrument in that sound, but they're damned sure gonna add more.

Motorhead would flirt with thrash tempo once in a while, but if you mentioned it, Lemmy would throat punch you. He insisted that Motorhead's calling card was playing rock n roll - as stripped down to the bare metal as he could get it. No costumes, no flourishes.

See, based on what you and utee said, I'm more inclined to put Metallica in with the heavy metal bands instead of "thrash."  Unless I seriously misremember their catalog, there's not much in the way of blast beats there, and they definitely were more melodic and subtly borrowed from Blues music often times. 

But, there they are, popularly thought to be one of the "Big 4" of thrash. 

Seems to me like they have more in common with Maiden, Sabbath, etc. than Anthrax. 

I'd agree with that, and that's why at the time I preferred their music over the more thrashy bands like Anthrax and Slayer.  Of course, that's before I had to begin boycotting them in the 90s when they decided to sue their own fans.