It just has some of his most powerful singing in it, lost of powerful B4s and all. He sounds very good in it too. I'm not sure if I'm right, but I recall Sir agreeing about it being one of Cobain's best.
He isn't doing anything there that he doesn't do better in countless other tunes. Even Mr. Moustache, from the same album, is more consistently impressive.
And just because they're B4's I dont think means anything.
Big Long Now has much more variety, with the contrasting vocal styles in the verses and choruses, and is just as intense and powerful.
"Smells Like Teen Spirit" requires more control, features a wider range shown and generally sounds better with Cobain's highs being much more powerful in it.
If there is wider range shown, it isn't by much at all. If anything, he goes lower in Pennyroyal Tea, so it all balances out.
And control? No way, SLTS is just reckless rock screaming. Nothing 'controlled' about it.
"Lithium" is like "Frances Farmer Will Have Her Revenge on Seattle", just with a much more impressive rasping technique. Cobain's rasp technique is actually pretty impressive in it. He never nailed the sound live, iirc. (And I have yet to hear somebody nail the sound in general)
He's doing more difficult things in Frances Farmer though, like actually sustaining
those raspy notes, instead of just briefly hitting them."I miss the comfort in beeeing saaaaaaaaaaaaaaad"...