Baphomet and Grazz'zt both seem to reflect the "Evil Dionysus" archetype right now. I almost want to give it all to Baphomet, beastly cults wearing dessicated livestock's heads, performing Hostel on the peasantry, and partaking in orgies, cannibalism and bestiality (this might be a great place to differentiate minotaurs made by biology and minotaurs made by the actual copulation of humans and bulls in violent, unholy rites). Make Baphomet the reptile brain in each of us, the base animal, and the untempered Rage and Passion which drives us to do bad things (with or without our knowledge- how many people these days black out during brutal acts, or how something just came over them). We'll never shake him from our collective unconscious, but civilization, justice, strong communal ties are his bane- torches against the the night of our unreason and filthy desires. His is not a cult of great aims, but can be meticulous planners in order to achieve their grisly catharsis, which may give a little insight into how Baphomet actually is different than Grazz'zt. That lord wants to twist the knife in your guts while you smile and thank him for it. Baphomet just wants your screams.
If I have any gripe, its the possible implication only three lords might get covered in the MM1. That's considerable, given the past, but the list of quality demon lords is long enough and varied enough that three only begins to cover their breadth and complexity. I'm a real sucker for them, especially if they're going to begin to cover what I think they always were meant to, every primeval, chaos, cosmic, and eldritch-like entity in the D&D universe. We should take Demogorgon's Cthulhu cue and nix a lot of pointless subdivisions of chaotic evil being and throw them all back into the primordial stew of the Abyss to hash it out as a lord or prince in the absolute nadir of the universe.
And how about a demon queen in the mix, too, like Oublivae maybe? She is so cool.