Having an expensive stereo in a bare, empty room is kinda like using the finest oil paints and natural hair brushes money can buy to make a painting on a piece of old newspaper!
I think room treatment has an effect not only on the physics of the sound, but on your mood. If the room isn't comfortable and inviting, why would anyone wanna sit and listen to music in it?
Room symmetry is equally as critical I've discovered. This is probably the hardest thing to achieve unless you designed your own home that is. Most living spaces are not condusive to hifi matters. I know that my living room sounds worse than my spare bedroom, which I am in the process of redoing with foam treatments. (ran outta money!) The living room is very uneven and has three openings which I cannot control. There's also more furniture in there for sound to bounce off of in bizarre ways. The bedroom can be totally closed off though, and this makes a positive difference. The speakers should be lined up as centered as possible within the space.
I don't have any bass problems yet, but there's plenty of ugly slap echoes in the untreated room. I spent a fair amount of change on Auralex foam. Visually the spikes are right up my alley, but I can see how many folks would not want it in their house, not to mention spousal issues. Absorbing all that "ambient noise" really made the room feel cozier even though there's hardly anything in it. You could probably do this cheaper by using some kind of fabric curtains or similar.
My next step is to try diffusion elements. I like the look of RPG skyline whatchamacallits, but those nutty things are like 400 bucks a crack. Give me a break. I'm thinking a nice hot wire cutting jig and a few sheets of styrofoam oughtta do the trick. Ahh yes, another unfinished project to add to my list!

Auralex has a great number of cool items, but like I said they are a bit spendy. The "Metro" stuff is a bit more conservatively styled.