So much stuff here - pity I'm leaping in late, but there are a few things I really wanted to respond to;
1. Agree that Reign In Blood is THE definitive benchmark for a thrash metal album, and yet probably the finest example of that genre, particularly in the drumming and originality stakes - no-one sounds like Slayer except Slayer from the get-go, yet RIB is the ultimate consumation of their sound and style married with perfect production.
2. Metallica evolved thrash from pretty much the minute they started playing it - they were never a pure thrash band, even on KEA. There's potentially a huge amount to discuss here - but I'll nutshellise it by saying that Metallica not only evolved longer song formats but had this unique way (thanks to Burton's supreme improv skills) of evolving riffs as pieces progressed in a manner similar to Classic Prog rock. The number of pieces this happens in is mind-boggling (but entirely in their first 4 albums - the best, IMHO). Listen carefully to Fight Fire With Fire, which is a great example. Each riff is a mutation or bastardization of earlier riffs - there's nothing wasted, everything is recycled and fitted into a uniquely evolving riff strategy. As far as I can tell, this strategy and compositional method was unique to Metallica, and pretty much demanded longer song structures in order to make it work.
3. Neither AC/DC nor Motorhead had a single sound that they simply stuck to for decades, even though it might seem like it!
AC/DC's early hard blues sound evolved fairly slowly over the first 4 years from the almost Glam Rock sound of "Jailbait" to the much harder sound on "Powerage" - but "Highway To Hell" is a completely different animal. AC/DC became brutal for 3 albums, and "Back In Black" is the pinnacle of that brutality - not in the sense of death metal, but in it's own right. "For Those About To Rock" is the beginning of them settling into a groove, more or less - but Angus never sat still - the evolution is surprisingly subtle, given AC/DC's musical style, but little things like the intro to "Thunderstruck" show that the band were not prepared to simply churn out 3-bar standards as Status Quo did.
Motorhead changed many times in their early years too; On Parole might have deadened the impact of Punk Rock, had UA had the guts to sell it when Motorhead made it in 1975. It's a brute of an album that kicks your ass from the seat of a Triumph. The follow-up, their self-titled album eventually released on Chiswick (or was it Stiff???) is lame in comparison, even though it's mainly the same songs. "Overkill" is a quantuum leap - this is a new Motorhead with a wide arsenal of musical styles. "Bomber" is a continuation of the "Overkill" path, but "Ace of Spades" is entirely new - an even more brutal yet slicker beast. "Iron Fist" continued the AoS style, but with added speed, then a complete overhaul for "Another Perfect Day" - the replacement of Fast Eddie Clarke by Brian Robson highlights the bands melodic side. IRC, "Orgasmatron" was next, a darker, more doomy Motorhead, as they suddenly realised that they weren't the fastest band in the world any more.
There seems to be a common pattern here - a bands first 5-10 years seem to be the most creative, then they slip into a style like a comfy pair of jeans and just churn out the product and watch the cash come in. Or am I getting cynical?
Just my $0.02...
