No I didn't experience it when it came out, but I can still make a judgement as to whether or not it lives up to the reputation it gets nowadays, especially compared to other albums from the same time.
Let's take just three other albums from 1986 - Pleasure to Kill, Darkness Descends and Reign in Blood. Without Master of Puppets, would thrash still have been in a great state at this time? Definitely.
Was Master of Puppets even bringing anything particularly new? Some melody you might say - perhaps, but not even as effectively as Ride the Lightening, and that's not to mention the lack of really THRASHY riffs. Several Master of Puppets tracks can be thrown away in my view.
And as for taking thrash to places it had never been before - well as mentioned above, it certainly didn't usher in any greater aggression or extremity like other 1986 releases did, which suggests you mean that it opened thrash up into the more mid-paced, accessible sound which Metallica (amongst others) would go on to refine into bland 90s hard rock.
I think thrash would have been perfectly healthy had Master of Puppets never existed. Metallica had already had their biggest impact in their formative years (which I maintain a grudging respect for), inspiring many other bands to play fast and heavy, but many of these other bands were the ones who took thrash to its most exciting lengths, and led to the evolution of death and black metal.
