Oscuro's Oblivion Overhaul
Amongst things, what it does is COMPLETELY remake the enemy and loot leveled lists, by hand. He grants min and max caps to almost all enemies in the game. Weaker type enemies will cap out at certain level, eventually becoming total cannon fodder. Most medium and strong enemies also have max level caps, but also might have MIN level caps, meaning that instead of having pretty much all enemies steadily level at you, you can (and will) potentially find enemies with a min level WELL above yours. So yeah, now there will be locations and caves that are way too dangerous for you at first, etc. Logic rules apply. Roads and light wilderness will hold easier enemies. Remote mountain tops and deep valleys will hold greater challenges.
Likewise, loot leveled lists are now completely redone as well. Regardless of your level, you WILL not be able to complete a set of Daedric Armor by wiping out a randomly spawned Bandit NPC mob. Steel starts showing up earlier. Iron and Steel will always make up the brunt of what you'll find. The stronger you get, the chances of finding elven, dwarven, ebony, glass, mithril, daedric, etc., loot (slightly) increase.
Etc.
Edit - Speevy. I prefer a rollicking good adventure too. Oblivion borked a couple of elements that were always part of TES adventuring. The OOO mod merely restores something that was always part of TES, and was destroyed in Oblivion.