Damn... I remember writing a humongous post about this exact topic a while ago, but I just can't find it anymore. That search function is useless!
Anyway, let's see what little I still remember from all that rambling I did about environment/ visual level design:
1. Avoid large, empty rooms, looong empty corridors and large planes with obviously tiling textures (walls, floors, ceilings). Break them up whereever you can by using decals, carpets, piles of fitting small objects (books, papers, rubble, lamps on a wall...) or simply by using a more interesting room shape.
2. Avoid placing all objects perfectly straight or in straight rows. (Unless maybe it's a throne room... every rule has its exception ^^) Rotating things subtly or furnishing a room with a bit more creativity than "everything straight along the walls" can go a long way towards creating a more organic, lifelike composition.
3. When placing large objects in a level, "integrate" them organically into the environment by placing fitting smaller objects around them. otherwise they'll stick out oddly. In general, try creating "object islands" around several large, eye-catching objects in a room instead of just spreading all the details around evenly. For example if you have a bookshelf in a room, put some bookpiles around it, maybe a small carpet in front or a comfy chair with a little table for reading and a lamp - this would create such an island.
4. When creating a room, think about its purpose first. Seems like a no-brainer, but I've often seen piles of storage crates and flour sacks in libraries or shelves with precious looking books in storage rooms. Stuff like that can look really weird on an almost subconcious level...
Last but not least: Make your level readable to the player. Think about where you want him to look, what you want him to notice first and what you want him to do. This is the most difficult part of level design and you'll have to really study how professional developers did it. (Valve for example is amazing at this...) I've written a bit about it
here (third post) , if you'd like to have a look. It's pretty noob-ish compared to what the pros do, but it might give you an idea what I'm talking about
