One of the major points on both e "Whats wrong with 4e" and the "whats right with 4e" threads is the separation of rules for NPCs and PCs. So lets discuss.
Different, different, different, different, and for the love of grawr, please different.
As a DM, I would like to craft my baddies quickly. It could be the rules such as in Spycraft 2.0, where you get a quick-put-together set of rules to make NPCs, or it could be "just do what you want". Either way, please let me put them together quickly.
As a DM, I would like to craft my baddies (NPCs and monsters alike) for coolness and uniqueness. One of the first 4E games I ran the baddie, on hit, could wrap his chain whip around the character and slam him into his armour spikes for extra damage. Very cool, flavourful, exciting and I didn't have to worry about how to build it.
As a Player, I want cool things to pop out and surprise me, and maybe even inspire me on how to go about doing that myself.
As a game world, I want the participants to be amazed by what happens, and not stop and say "Wait a minute, what mechanic did he use for that? How is that possible?" (which often happened in our 3.X games). 1e style immersion (that has worked well in the current edition).
Provide guidelines to give starting DMs a baseline to match PC's levels, with sidebars about more and less powerful opponents and encounters, but please let them be built on different rules so that the villains can do wild amazing things that have me and my PC grunt and push to the limit to overcome and savour the victory.
peace,
Kannik