Many good points have been mentioned regarding treatments. I spent nearly three months modelling and experimenting with acoustical panels at the reflection points and building bass traps. Anyone who wants to improve the sound quality of their HT room should carefully treat their environment. Its a highly effective way to optimize sound reproduction. It doesn't have to be expensive and it can be tastefully done as Randy has shown.
Existing furnishings like bookshelves at the rear and side reflection points are excellent for sound dispersion. Rugs on the floor or blankets on hard funiture are easy quick treatments. Heavy velvet curtains around the sides of the screen are another easy treatment which adds to the theatre like look.
And the more bass traps the better. I have four floor to ceiling traps in each corner as well as several at wall ceiling corners and that has been the most significant sound upgrade to my HT. I may build several more though the room is already "studio-like" according to most visitors.
Taming the first and second reflection points is an important objective if one is to achieve excellent sound in an HT. I agree one has to be careful not to overdo treatments along the walls but placements at the mirror images along the walls and ceiling do indeed get rid of smearing from the main channels as a result of delayed sidewall and ceiling echo. That phenomena is different than optimizing spaciousness through off axis dispersion, an important attribute for the side/rear surrounds. I found coverage of about 25% of the wall area with teatments did much to tighten up and clarify bass and mid-range. The upper mids and higher sounds benefited greatly from absorption at the first reflection points.
For strictly stereo listening I have opted for a spacious vaulted room with far away reflection points so speaker placement/toeing and listening position are the primary acoustical determinants.
Oh yes. A strategically placed rug, armchair and bottle of cognac can also be very effective in dulling those reflections.
