I like the idea of magic weapons and rings and armor etc. being exceptionally rare - so when given or finding an item it is a major experience.

Does anyone know if there are mods that make the world harder and more "realistic" in terms of rarity and reward?

J

Among its various changes to the game, http://www.tesnexus.com/downloads/file.php?id=25078 features an option to reduce the chance of finding magic items. Everything aside from the dynamic barter gold is completely optional, so if you don't like some changes, you can disable them. If you hate the idea of the dynamic barter gold, however, I'm not sure that there's a standalone option for what you're looking for.

merchant and loot overhaul will also take away weapons and armors from containers and give em to merchants instead. not to everyone's taste, but an excellent mod. if you use it with enhanced economy which makes money a scarce commodity, buying high level gear won't be easy