I probably have to high of standards. But none of the D&D works on the underdark capture for me the wonder (or danger) of cave passage or the sense of infinite adventure that's found in something like Verne's 'Journey to the Center of the Earth'. The underdark should be alien, fantastic, and harsh. Not harsh just because everything roaming around in it is deadly, but rather everything roaming around in it is deadly because its so harsh. I'd like to see an underdark where everything down there on is the verge of starvation, and where wars are fought over patches of edible mold and mushrooms and caverns that allow thier cultivation. I'd like to see a Drow with a truly chaotic society, say something more like the culture of nomadic lion prides, than the hyper organized lawful predictable culture they've come to have. I'd like to see an underdark of alien wonders, or at least one which seems to be actually in three dimensions.
Nothing truly satisfies, but I don't think you could do much better for inspiration than the short description of the 'Deep Earth' in the 1st edition Dungeoneer's Survival guide.