An open game wouldn't be particulary open if it got lots of areas you can't go to because the enemies there have too high level. If certain areas had certain enemies and you could only follow one path, it may as well be a linear Bioware RPG like Jade Empire or whatever.
Gothic 2, for example, had this problem. You can't really explore the game world in the beginning of that game if that is what you want to do. If you try it, you're dead because there are extremely powerful enemies in all the directions except the one path you're supposed to take. Which totally kills the exploration aspect of the game, for which many TES fans are TES fans to begin with.
Which makes the game very linear in nature, and not really that much different to various linear Bioware games like Jade Empire. But funnily enough, Mass Effect 1&2, although fairly linear compared to RPGs, have the same extreme level scaling as Oblivion. Yet I never seen any complaints about that, kinda interesting.
That said, quest related enemies and bosses are a bit different, and not really a part of the "game world enemies", and could be treated in a different way. This is kinda how Morrowind did it, the game world enemies were level scaled, while the main quest enemies were not.