No melody. Have you ever noticed any Keyboard parts in any Grunge song?
Weak (and stolen) guitar riffs. Guitar riffs in Grudge are very simple. And most Grunge guitarists were mediocre and nobody remembers them.
Forgetable Basslines. Do you remember any basslines from any Grunge song? If you have ever tried to learn to play the Bass did your teacher ever suggested you any Grunge song?
Boring vocals. Grunge vocalists sound like, they don’t enjoy what they are doing.
It’s all about attitude. But even this attitude is absolutely pathetic, since even Grunge musicians themselves were branding themselves as losers.
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I can only speak from a singer's perspective, but there were some talented vocalists in that era. Chris Cornell (Soundgarden/Temple of the Dog) had 3.5 octaves from D2 to A5, and at his peak could hit strong G5s and G#5s fairly easily. His vibrato was strong, though he didn't use it much. His upper register is what he's best known for, but he hit some impressive lows as well.
Layne Staley from Alice in Chains was better known for his mid-range singing, but had roughly the same amount of range as Cornell. On some earlier demo songs, he was effortlessly hitting a lot of Eb5s to F#5s. His vibrato was perfect, even on demanding parts (Bb4s on the final chorus of Man in the Box). Despite not going outside of his comfort zone very often, he could very smoothly pass between registers, like on the intro to I Can't Have You Blues.
There are others of some note, like Skin Yard's singer, whose name I can't remember; who sang very similarly to Cornell, and Scott Weiland, who had far less comfortable range than some of the others mentioned, but still was a very solid singer nonetheless.
I do agree that a lot of grunge, especially the most popular stuff like Nirvana and Pearl Jam, is largely talentless. It sounds more like you just don't enjoy the music, which is fine. But as I mentioned, grunge was not entirely void of talented musicians.