There's good and bad immature, true, but the bad on Show No Mercy is hilaribad. Tormentor is probably the best example. The intro riff admittedly does have a cool Candlemass-y doom sound to it for about the first two seconds of it until Lombardo fucks it up with his terrible drumming, and then it descends into dance land with that cheesy Do-do-do Do-do-do riff. Seriously, that riff makes me want to do this:
, mockingly. I laugh every time I hear it. Then come the lyrics, which could be glam metal if you removed the occasional reference to violence and rape. Seriously, it's about as thrash as Live Wire. Not that it has to be thrash; Jump in the Fire is an awesome song despite being a bit rockish itself. Tormentor is just shit.
And I realize that's just one song, but a lot of the album feels like that to me.
Die By His Hand (chorus aside), Fight Till Death, and Black Magic are great songs but a lot is forgotten-demo-band worthy.