Large seamless world, props and items get loaded differently, those can be detailed and have much variation. If the world itself doesn't, it's not that bad. You don't just walk from a forest into a desert... and 20 minutes later in a jungle. Atmosphere should be set by the props, buildings and behaviour of npc's rather than textures.
For example, if evil has affected a certain region, the enviroment doesn't get darker, it does not have to be clouded, raining... but npc's act frightened, they mutter and peer as you walk by, tools are just laying around where in not affected regions everyones friendly, smiling, organized and at work. In bad areas you don't hear much birds, some crows but it still can be bright bright sun, green grass, leafy trees.