Broadband sound absorption is a widely acknowledged way to improve the sound of almost any untreated listening room. Properly placed sound absorption devices improve sonics by reducing or eliminating midrange and high-frequency sounds that arrive at your ears after reflecting off a room boundary (e.g. walls, ceiling, uncarpeted floor). This reduces slap-echoes, comb filtering, and other objectionable, generally muddy, acoustical properties of untreated listening spaces. Commonly cited improvements include a smoother frequency response and cleaner, more focussed stereo (or mulitchannel, as the case may be) image reproduction by your loudspeaker system. Also, if one is careful to construct absorption panels of the proper thickness and materials, one can also help tame room modes by absorbing or "trapping" reflected bass signals. This reduces the intensity of low-frequency standing waves that are created in-room which often skew the bass reproduction of a system very significantly, especially if speaker placement options are limited by physical or aesthetic constraints.
That is all well and good. However, one should be careful to avoid using too much absorption in a given space or the room may sound dead and unnatural. It is therefore typically recommended that a balance be struck between acoustical absorption and diffusion. Diffusion spreads reflected sounds evenly throughout the room to create a natural acoustical ambience without coherent reflections that cause the unwanted artifacts described above. Bookshelves, plants, and convex curved surfaces are simple devices that can be employed as diffusors, and there are an array of commercial products as well. Acoustic phase grating (1D quadratic-residue, or QRD, diffusors (patent)), Omniffusor™ (2D QRD diffusor (patent)), and Skyline™ diffusors (patented as the 2D primitive root diffusor) are all examples of commercial designs that can be purchased (or copied - for personal use only, of course) and applied to meet the goal of improving acoustics without over-deadening a listening space.