One of the biggest take-aways from Justin’s Calibrating Your Expectations is the meaning of levels. He says that he began writing the post in part to address all the people who said that “D&D Can’t Do Conan” because making a Level 20 Barbarian gives you a guy who can do things the actual Conan never could. Or “D&D Can’t Do Einstein” because a Level 20 Expert would have way too many hit points for a frail old math geek. The argument he makes is that people are looking at Level the wrong way, and they’re expecting Level 20 (or Level 5, or Level 12) to mean something that it doesn’t.
Justin makes the argument that pretty much everyone you’ve ever known would be a Level 1 character. Really exceptional people might be Level 2 or Level 3. Level 4 characters are some of the most talented and accomplished people in the world, and Level 5 characters are the people who get written about in history books. From there he calls 6th level superhuman, a 10th Level character is challenging gods to contests of skill, and a 20th Level character is essentially a god themselves. He bases his argument off of skill bonuses available at level 1 and DCs attached to certain activities. But there are other clues, too.
A 5th Level character is taking on manticores, trolls, and young dragons; the exploits of Beowulf. Heroes of Greek myth fought a minotaur (CR4), a hydra (CR4), and Medusa (CR7). At Level 10 characters are fighting Greater Elementals and huge extra-planar spiders. Above level 15 characters a fighting high-level Angels and Demons, and when they reach level 20 they are literally beating up gods and taking their stuff. Think that those who have made creatures higher than Level 20 are well-meaning but misguided, and I personally believe that Zeus himself can be built as a Level 20 creature. (It gets futzy when you’re talking about CR versus Level, though.)
The long and short of it is as Justin puts it at the end of his post: the 3.X system expects that you’ll move from one power level to an extremely different power level as you level up, but people expect there to be a much more uniform performance from Level 1 to Level 20. They bend over backwards trying to make the system fit that expectation, so that a 20th Level character can be treated as King Arthur instead of as Thor. (As an aside, this is precisely why the trend of D&D 5E worries me; they’re trying to flatten the playing scale so that a 20th Level character is still threatened by orcs. You lose a lot of variety in what the system can model when you do that, and it isn’t necessary.)
And of course, these kind of expectations are really harmful to the game. If you expect that Aragorn is Level 15 instead of Level 5, then that colors what sorts of adventures you can have at low levels. You spend the first 5 (or more) Levels of D&D killing rats and goblins and bandits, instead of leading armies, storming castles, and fighting Nazghul.