1. I think I agree, but I'm not sure. Do you mind going into more detail about the differences?
The rules should permit PCs to do anything NPCs can do and the reverse as well. This thread is about rules. In 4E there are differences on the mechanical side that treat PCs and NPCs differently. I am opposed to that.
If you want to have an npc with a cyber arm and have it be impossible for PCs to have one, then fine. But if the system rules make it possible for the NPC to have the arm there should be nothing in the rules preventing a PC from having the arm. But there may be plenty of story / setting reasons. Maybe this guy fell through a time door and there is no means for anyone else to copy him. Maybe the evil cybercorp just unleashed the technology and repeating it will take 20 years of research, if you want to wait.
As another example I could easily take all arcane spellcasting away from 3E PCs without changing anything at all mechanically. Off the top of my head I could create a world in which there is a mystic barrier that prevents magic from working. However there are heinous acts which lift that barrier. As long as those acts have been completed within the last 24 hours magic works in a 500 mile radius. There are vile cabals constantly performing these acts so that their lords can practice magic. But everyone knows that all magic is founded on pure and constant evil. The campaign is centered on fighting these evil cabals one cell at a time.
Now mechanically arcane spell casting works EXACTLY the same. But functionally it is limited to npcs by the story.
In 4E there are differences that the very mechanics bring to the table before setting is even mentioned. I would be inclined against any system that did that.
2. What would you lose by having something simple enough to build on the fly or in your head?
Be fair now.

That is different than what you said before.
I already have a system that I can build off on the fly and in my head. Just that and nothing more would lose me nothing. But there is some system mastery included in this. I'm ok with that.
But if someone else considers 3E to not meet the standard of "simple enough" then I don't want MY system "simpled down" to meet their standards.
I would LOVE to see a better mousetrap. If you can build an even easier system than what I have now and still have every bit of the MECHANICAL richness built in then I'll happily embrace it. I consider that wishful thinking. Until demonstrated to the contrary my answer here presumes more simple comes with a cost in value in exchange for zero added value *to me*.
3. I agree. I think a table of ACs based on narrative considerations - "AC 18: Steel covering the entire body, with few weak points" - would be my preference. Not only because it's simple but also because it's easy to give players information about what AC they are facing.
Why do the players need to know? Isn't "Steel covering the entire body, with few weak points" everything they need?
I like lots of different ways of getting there as well. If your Dex 10 Full plate guy is 18 the exact same guy except DEX 12 is 19. And the mage armor 18 dex guy is also 18. Not clear from what you said if you were leaving that out by design or not.
But what I REALLY don't want is to have the AC the NPC influenced by the PCs in any way. I've complained about that in 4E before. People have praised the idea that the bare chested pirate and the steel covered black knight have the same (or close to it) AC and that AC is based on the level challenge they are expected to be.
4. I think this is a big disagreement, unless I misunderstand you. I think it's a question of what the goals of the game are. I'd prefer it if the game gave the players a choice of how much risk they were willing to face, knowing that the greater the risk, the greater the reward.
Sounds like there is a huge difference here. Characters in stories don't choose between the troll with a bag of 750 gp or the 7 orcs with a bag of 500 gp or a the necromancer with a magic sword.
If the troll is eating kids from the village the party goes out to kill the troll.
Even if the campaign is a pure "bounty hunter" thing, it seems irrational to me that there would not be uncertainty on rewards for known foes and then unexpected challenges that along the way. And this gets back to #1 anyway. Even if there were clearly defined "bounties" those should be 100% narrative based and not be even a consideration in mechanical design.