So why isn't there more discussion of it? Mixed in among the reams and reams of treatises on Tolkien, and the smaller-but-still-notable analysis of Robert E. Howard, you'd expect to find at least a little something. Two picaresque-prone heroes who wander about a magical world set on the inside of a sphere (thus enabling you to sail to the far side of the world and encounter the sun, moon, and stars), and Death rather ineptly sets traps for them because they've become too powerful and famous? Leiber's fantasy is more fantastical than Howard, or Tolkien for that matter, because he didn't try to present a realistic world. Nehwon is a dreamscape, not a landscape.
Probably if there's a weakness in the Lankhmar stories, it's that they're uneven. Unlike Howard's tales of Conan, which were all written over 5-10 years, Leiber wrote about Lankhmar in fits and spurts for three decades, so naturally they're going to be "all over the place" in terms of quality and subject matter. (As far as my own preferences go, I tend to enjoy his earlier, more purely sword-and-sorcery stuff, to the later weird sex and odd rambling. There's something to be said for writing for the '50s publishing mentality over that of the '70s.)
I wonder if maybe it's because Leiber seems so much more contemporary than either Howard or Tolkein. (The earliest Lankhmar stories were written shortly after Howard's suicide and predate the good professor, but Tolkien deliberately used a more archaic style, and Howard just couldn't stop himself from writing like a cross between Homer and Hemingway.) Leiber's sly satirical style and more modern word usage make his wild images seem more like a head trip than an epic fantasy, so maybe he doesn't have that "on a higher plane" feel that a lot of fantasy readers love so much.
Or maybe he just hasn't been dead long enough. It has barely been 12 years, after all. I dunno.