
Okay, so people swimming up waterfalls is silly, or walking along clouds, or hanging barehanded from ceilings? Well, you'd better not allow spells like Wind Walk, Spider Climb, or . . . well, some sort of swimming spell.
And level 20th+ doesn't necessarily mean Norse or Greek or Hindu heroes. Have any of you seen any anime series like Slayers? How do you handle someone like Lina Inverse who can shoot off a nuclear explosion pretty much at whim? It's not that the characters in Slayers are god-like. Not at all. Rather, it's just loose, high-powered entertainment where the characters are nearly the best at what they do, so their villains are equally powerful if not moreso.
A lot of anime characters would be epic-level, but I bet that if you're watching the anime you wouldn't say, "That's so munchkin." Rather, you'd be enjoying the fun, and liking the fact that though they're really powerful, they can still have foes, can still have emotional development and conflict.
I myself do kinda think that spell level increases should slow down, so that you only get 4th level spells at 20th level, but you can cast a lot more of them. That would let levels scale more easily.
Still, just think of it as cinema or animation, and you'll be ready to enjoy the game. Your PCs aren't all powerful, just more powerful, which lets them show off their stuff more readily. It should be fun.
(Mind you, I've never run a game past 18th level; the cleric of time casting time stop while hasted and shooting off 15 spells from a wand [and quicken spell] kinda soured me to the fun of high-level games, but I'm coming to like the idea again. I'll just be banning time stop and haste and all other time spells.

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