Probably his ability to combine so many different styles, moods, themes, etc. I think Buk is unique in that he is violent, sacred, low-class (beer, bums), high-class (Artaud, Mahler, Shostakovich, fine German white wine), tragic, nihilistic, absurd, extremely funny, witty, etc. I mean is there another writer you can think of that can do all of that?
Usually it's one or the other: are Dostoyevsky or Sartre very funny? I think Beckett's "Murphy" is fairly funny, but not as funny as Buk. But there is perhaps a very specific American humor in Buk.
He is witty, quick, delivers lines with an impeccable sense of comic timing.
Sometimes I wonder: is Buk a comic writer mainly?
But no, because then you think of those horrible, depressed, suicidal poems in "Love is A Dog from Hell" or so many other anguished moments in his work.
He's both. That's why he's unique and great.