Thash in itself is bloody basic, just play super fast, heavy and aggressively and you`ve got thrash. I still think Slayer`s Reign in Blood the best example of a pure thrash record, 29 mins of pure hell.:bonkhead:
I think Metallica quickly evolved thrash, slowed it down and even by Ride the Lightning had incorporated melody and longer song structures. Puppets though, was just the pinnacle of all this and their masterpiece, as for where they were going to go, I suppose they were faced with two obvious options and that was to go more complex than Puppets which they tried to with Justice or just get commercialized which they did later on with the Black album.
Totally agree with you about the evolution of a band and once you`ve achieved the pinnacle of your sound its time to move on. I think the yarRABtick of any band out there, should be the Beatles because their evolution from the poppy "Love me do" to the albums of Sgt Pepper and then onto Abbey Road are quite simply amazing and all this in just 7 years!!! Whereas say AC/DC, have been banging out the same sound for like over 30 years with little change!!!
Also as far as thrash goes, I was never actually a fan of James Hetfield`s style of singing and far preferred that of either Dave Mustaine or Tom Araya but I admit his dominant, loud voice was most suited to Metallica`s sound, I think though, that on Load his voice had also evolved along with the groups change in musical direction and think is vocal display on that album to be very good.
All in all I think Puppets is their best album but Load suprised me, that they could do a hard blues rock sound as well as they did.