As far as encounters being different from one level to the next....
We already have a level-based game that keeps it's sweet spot from one level to the next all the way to the end of the game. Mutants and Masterminds.
And the difference between the levels in Mutants and Masterminds is pretty remarkable. At PL 6 or so, you're a street fighter level hero, beating up thugs. At PL 10, you're like most heroes in a comic book, fighting thugs as well as super-powered characters. At PL 16 or higher, you're going into outer space or other dimensions fighting stuff there.
As far as difference in experience, in M&M, the higher in power level you go, the wider the variety of things that happens. At the low levels, you might throw a car or knock a hole in the wall. However, at higher and higher levels, you might knock down a building or take out the side of a mountain.
I would imagine that the D&D 4e experience would be much the same. As you progress in levels, you go from skirmishing a few kobolds to taking on a legion of orcs. You go from saving Farmer Brown's cottage to saving the whole village.
Also, the variety of things you are able to accomplish changes. As spellcasters gain more access to spells, they start being able to do more things on the battlefield and in the game, which leads to a wider variety of things that can happen.