There are several competing demands placed on NPC creation, some of which conflict. Pertinently, there is the demand for NPC detail versus the demand for speed/simplicity of NPC creation. These can be satisfied by using a tiered system of NPC creation, where the DM has the option of creating NPCs using the normal PC rules and classes, and a simpler system for the less important NPCs. The 'simpler' system in 3.X edition entailed using NPC classes, but that wasn't really all that much simpler than using PC classes.
Of course, important NPCs using PC-rules should remain, but there is scope for simplifying the lower tier(s) of NPCs.
My idea for doing so is to base the weak/simple NPCs on the racial traits and racial hit points (this is in effect a maxed racial hit die) as presented in the Pathfinder RPG option for starting hit points and not give them any levels. Instead, there would be NPC templates for the most common types of NPCs:
e.g.
Farmer template
Savage template (barbarian commoner)
Blacksmith template
Guard template
Thug template
These would have pre-defined traits and suggested skills/feats to make the DM's job easier, but he would, of course, be free to move them around if he wished to do so.
Such NPCs would be treated as first level (1 HD) characters (owing to their one racial hit die) for the purposes of spell effects and other situation where knowing the HD/level of the NPC in question is required.
The Commoner NPC class would thus cease to exist altogether (the fact that it exists in previous materials does not really hamper backwards compatibility - after all the NPCs using it are already statted in those older adventures, so their stats wouldn't change).
Experts, Warriors, Adepts and Aristocrats could be kept as NPC classes for a middle tier of NPC-creation. In principle, these too could be replaced by templates, but it would require a number of templates per class to account for the various higher power levels of those classes at different levels.
Having read the sub-headings for the forum, I have just realized that this is perhaps better suited to the New Rules forum, so I will re-post it there.