Regardless of low or high (>6) level, it comes down to the challenge of the encounters. One area where I think a lot of developers get trapped into the same old tropes is when they scale everything for high level characters. In ToB for example, suddenly every character you meet has >100 hit points, even the gate guards! Ridiculous. And that is probably also why all high level D&D adventures eventually end up in the Underdark, because there you can justify all sorts of high level characters walking around. I am not a fan of this kind of global level scaling as a means of creating a challenge. I liked the fact that in BG I your first level character has a chance to interact with a 15th level wizard (Thalantyr). I think I would also enjoy the converse, where perhaps my high level PC (>6) has to deal with a low level character, if the story was good!
This is a very good point with which I fully agree. I think this is part of a bigger issue in cRPGs and especially in recent years: the expectation that every single NPC or creature in the game is there for the purpose of giving the player something more to kill. It's bad enough that we now have people clamoring for the "right" to kill everyone and everything in a game. The next stage of this seems to be the demand that all those NPCs and creatures that one can kill should also be "challenging," so that one can pat themselves on the back for having done what they did "for the challenge of it" and not because of their psychosis. So when that god-level PC wanders across a puppy and they want to kill that puppy, the puppy had better be "challenging" so they can feel okay about killing the puppy.