I've never been happy with demons and other extra-planar creatures in D&D. By the time people started expanding the cosmology and inhabitants of the infernal planes, the "design" principle of "longer list of powers = more interesting monster" had firmly taken hold and it was pretty standard to make a more powerful creature, like a demon, by just adding more and more powers. Eldritch Wizardry lists six powers for Type I demons, eight for Type II, nine for Type III, and so on. Furthermore, several powers shared by all demons are at different strengths for each type, and some of the strengths seem random, jumping from 5' Darkness to 15' Darkness back down to 10' Darkness, to name one example. It's even worse in AD&D; each edition added more and more powers to basic supernaturals.
The problem with this approach is that you have to refer to the demon description not just once, but repeatedly during an encounter; there's no mnemonic sense to it all. Some of it could be easily fixed by consolidating powers (get rid of levitation, for example, because all demons have telekinesis as well, so they can just use that.) Some powers can be consolidated as power themes (use all fire- and darkness-related magic up to level 4.) Other fixes depend on the broad supernatural category (demon, devil, etc.)
Also, I have my own problem. I prefer the simple Law/Chaos alignment split, with alignment as faction instead of behavioral descriptor, plus I prefer the non-planar approach I've described before: "ethereal" and "astral" are states, not planes, and places like Hell or the Abyss are deep in the earth or on another planet. This requires changing some of the details for "extra-planar" creatures, eliminating some types that seem to exist just to fill a conceptual niche (demon vs. daemon vs. demodand, for example.)
I have some ideas on revamped "extra-planar" creatures that I will be posting during the coming week.