In regards to Reign In Blood, I think the only album more overrated than it is Master of Puppets. That being said, there is a clear distinction that Reign In Blood actually did something for the thrash genre rather than Lars pushing Cliff's hippie tendencies to the forefront of his band to fool moron journalists and critics into thinking Metallica was better than they actually were.
Even though there would be much better thrash albums to be made (even from 1986), Reign In Blood does deserve credit for at the very least, lighting a fire under the ass of the metal world. Love it or hate it, it truly did push the boundaries of metal much more so than Metallica trying to claim they were much deeper than being just a speed metal band (imagine the horror of that). It was an inspiration for not only thrash, but black, death, grindcore, and I actually think its underrated in that you don't hear it be given a whole lot of credit for bringing the punk and metal scenes together.
No my issue with this album is that there is quite clearly filler on it. All the people who claim "every note of every beat of every song is flawless." No. No No No No No. This album has filler. Repeat it in your head over and over until you have convinced yourself.
I also agree with Gordo in that quite a few of the songs are a bit underdeveloped. These songs are namely Piece By Piece, Necrophobic, and I'm also gonna throw Criminally Insane into that mix. I think people overrate Criminally Insane because that middle riff is so awesome they think they hear it and the beginning of the song and think it's the primary riff to the whole thing, whereas the intro is not quite as interesting.
I do think there are more good songs than just the first and last tracks; Angel of Death, Raining Blood, Postmortem, Altar of Sacrifice, Jesus Saves and the most underrated track on here Epidemic. Pay attention to the thrash break in that last one, simply fantastic. Obviously Angel of Death wins the knockout of the night, but that one can't be overlooked.
An album with no filler means that in this MP3 age where you can hear the "hits" anytime you want, the rest of the album, when you finally get it, stands up on its own. I became quite familiar with Angel of Death and Raining Blood as MP3s long before I got the album and the rest of the album just couldn't wow me like those two did. When I got Van Halen 1 I had long become familiar with Running With The Devil, Eruption/You Really Got Me, Jamie's Cryin and Ain't Talkin Bout Love. And that last one was my favorite Van Halen song ever for a while....until I got the album and heard I'm The One. That song, Atomic Punk, Feel Your Love Tonight and every other song tore my fucking head right off. And that was after following the same process as I did with Reign In Blood. That's how you can tell an album has filler. Reign In Blood has filler, though its more deserving of "greatest metal album of all time" than Master of Puppets ever was, because at least it influenced good ideas instead of bad ideas.
I could DIIIIIIIIE! Or worse, DIIIIIIIIIIIIE!