1) Opponents are always aplenty for the PCs. PCs can never run out of opponents. But for the "fixed" number of "named" NPCs I would not count the random encounter tables as relevant of the power level of a city/country. It is the "named" NPCs that matters the most as your players will need to interact with some of them for buying/selling/contracts for object making and many other possibilities. The nameless blokes meant to be opponents for the players do not matter one iota. Be it Greyhawk, FR, DS, Eberron or any other settings.
2) I partially agree. A lot can be explained by play expectations but not everything. The tone of Greyhawk is way different. The amount of High Level NPCs in Greyhawk is a lot less than in other settings. This makes high level PCs both an asset and a liability. If they are your allies, HURRAY! If they are your foes... DOH! A single high level fighter in Greyhawk can possibly take on an average castle by himself, imagine if he is with his fellow adventurers... (for an average castle, take a look on the Assassin's knot adventure. A 12th level fighter will slay everyone in there without having a serious trouble. Well, a well placed "hold person spell" might be problematic but if the fighter has the chance to slay them first... The rest would be a cake walk. For the standard castle in FR, go check on the Bloodstone lands supplement FR 9. Lots of high level opponents in there. Yet, Bloodstone is barely the equivalent of Furyondy and this is a nation besieged by a demi-god! Hell, even the supplement Castles! we could see the difference between the settings.
A high level PC in Greyhawk will bring a lot more respect than the equivalent level in the FR. Simply because the lord of the land is probably unable to take on the PCs by himself if nothing else. This alone should bring a lot of respect. In a lost place like Shadow Dale, Mourngrim was 14th level! And let's not talk about the others around (and do not forget Elminster himself...). Hell, Khelben Blackstaff had a special rule that he was always 5 levels higher than the highest PC... If that is not a power spike, nothing is. You don't see that in Greyhawk. Tenser in the Island of the Ape is of the same level as the PC. And the amount of treasure he pays them is astronomical! He knows he can't do it himself and if you take a careful read of the text that is to be read to the players, Tenser is respectful and careful in choice of words with the PC.
I think your points #1 and #2 run together in a strange way.
You say it doesn't matter how many "nameless" casters there are of any given level, because they are meant to be foes for the heroes to slay.
But, I think that same meta-game logic applies to things like Khelben Blackstaff's rule. It isn't meant for saying that if you encounter him at level 1 he is level 6, and by level 17 he is level 22. The intent is that he is supposed to be more powerful than the party, and if the DM wants to run a high-level adventure, then he needs to be higher level to still maintain his role in the fiction.
And, you have to consider those random NPCs if what you are saying is true. If a level 12 fighter can defeat an entire castle by himself, and the table says that you have a percent chance of running into a squad of 1d6 level 10 fighters.... why aren't they in charge? These guys could take over the entire city in a matter of hours, why are they nameless thugs without a purpose in the plot.
And I think, though I have no backing for this in research, that trying to find a solution for that is what led to FR's overabundance of important NPCs. DnD, by and large, is a game that lends itself to societies that are meritocracies (if not theocracies and Magocracies) because the average level 10 adventurer is a match for a squad of guards. A single mage has more personal power than many nobles in old times could have dreamed of having.
The solution FR tried, it seems to me, was to have enough big fish in the pond to say "these people are keeping the status Quo, that is why the level 14 mage you are fighting isn't running his own country"
Of course, this led to a new problem. Why aren't these people saving the day and maintaining the status Quo? Which led to new solutions. Some of which working, some of which not.
But, this is a fundamental problem with fantasy and superhero worlds across all media. Character X is powerful and intelligent enough to be a better leader able to keep her people safe than the people in charge, why isn't she in charge. Character Y is so powerful, if he wanted to be in charge, no one could stop him. Why isn't he in charge?
Sometimes people find reasons in the fiction, sometimes people are just as powerful as they need to be. If you need the character capable of casting Raise Dead in DnD, they need to be at least level 9, no matter if that makes narrative sense from the perspective of the world. I think this was also the impetus for the NPC Classes in 3.5, trying to create a new set of rules that goverened this part of the fiction, so there didn't feel like a big disconnect, or the NPCs having powers the players don't.
