I'm fine with some hacks to simplify the process, but yes, ideally. The rules should lead to knowable outcomes, where the players can make decisions about the potential capabilities of others from a shared set of norms.
Another thread that was brought up several times in the part.
I think they should be made like monsters and not PCs. They can have whatever the DM needs them to have. If you need the bartender to be able to cast fireball 1/day, then fine. You can make up any reason you need. I also tend to call my NPCs be other names than the classes so my fighter is a warrior of guard but might have fighter abilities or slightly changed ones like second wind on you or someone next to you for example.
You will also get some answers that say PCs should be able to get whatever NPCs can get to keep with game balance. If the bartender learned to cast a fireball 1/day, then there has to be a way for my rogue to get it as well.
i wouldn't strictly say they should be. they can be. there's no problem with building (especially humanoid) NPCs using PC rules. there's also no problem with inventing mechanics specifically for NPCs (and there isn't a problem with giving PCs a way to get their hands on said mechanics either).
If we are talking about humanoids that are similar people like the PC, then sameish. (For literal weird monsters it doesn't really matter.) For usability stuff often benefits from some streamlining, and a lot of NPCs are not as gifted than the PCs so they don't need all their capabilities, but they should use same "building blocks." So an NPC assassin has the same sneak attack feature than the PC rogue does, casters have spells chosen among the spells available to the PCs rather than some unique spell like powers etc.
There can of course be some exceptions to represent rare forbidden techniques and forgotten ancient spells etc, but I think the stock NPCs in MM and the like should follow the principle I described above. It is easier to use as the mechanics are familiar, it makes the world feel more consistent and predictable, as well as avoids the questions of why the PCs cannot learn the NPC powers.
I also like if the classes are things that have some cultural and/or metaphysical existence in the fiction, rather than just arbitrary power packages, and having NPC members of the class to be around reinforces that.
Not really. As in the original game, monsters and PCs having different stat blocks and different rules (or no rules, in the case of monsters/NPCs) for determining their stats is fine.
And, an NPC could either be, like, a DMPC companion to the PCs, built just like one of them, a high-power NPC mentor/check that goes far off the rails, or a monster stat block.