There are two aspects to room treatment. One is to improve the balance of the frequency response and minimize room modes, mostly on the low end. This generally requires both diffusion and absorption. Not all rooms need this, and it can be addressed in different ways.
The other, and more important one is to reduce room reverberations in the mid and high frequencies that smear the sound, and add the sound of your room to the sound coming out of your speakers. This can be accomplished with absorbent materials. Most rooms can benefit from this and can take your sound quality to another level. It doesn't have to be complicated. Furniture, carpet, can help but placing 4-6" deep absorbent panels strategically works wonders. And they can found with all sorts of decorative looks.