Part of it (and I'll admit I'm being really hipster at this point) is just having been there from the beginning.
It's hard to look at people dumping all this praise on later day "big 4" albums, and completely blowing off the earlier work. When my friends and I were first falling in love with thrash...there was no Master of Puppets, or Reign in Blood, or Rust In Peace, or Among the Living. (and there was *absolutely* no Black Album, Seasons in the Abyss, Countdown to Extinction or State of Euphoria). We were 16 years old, and we were so in love with Ride the Lightning, Hell Awaits, Killing is My Business, and Spreading the Disease that we had pretty much given up listening to anything from old school bands like Judas Priest and Ozzy. We loved it BECAUSE it was so much more extreme than what anyone else was doing, it just made us want more. So we dug deeper, and found bands like Sodom, Kreator, Destruction, Venom...and basically anything on the Combat, Metal Blade or Megaforce labels.
So hearing our beloved *extreme* acts release things like CtE and TBA was a major let down. But RiP is good...I just happen to like So Far So Good better (my personal favorite Megadeth album)