Rectangular footprint with no dimension a multiple of another.
This, basically. Make it a sealed room where air cannot escape. Use constrained layer damping in the wall construction to prevent energy from leaving the room. If you're really dedicated, get real time analysis equipment like a measurement microphone in order to determine the kind of in-room acoustic balance you are getting. You may have many bass issues which need may damping.
Past that, the factors which contribute IMO would be dictated by your budget. At the very least, you should factor in three or four subwoofers, three potent front speakers, and at least two lesser surround speakers. How dynamic and expensive these things need to be is dictated by your seating distance and room size.
A large theater room needs more powerful speakers, bigger subs a bigger projection screen, etc.
Personally I think something like 18 ft wide, 24 ft deep, and 12 ft high are good dimensions for a two-row theater room. Place the seats about 10 to 14 feet from the screen/speakers, place the side surrounds be the rear row of seats (or two rows of side surrounds running simulatenously beside each row) along the side walls, and rear surrounds along the rear wall. I wouldn't bother with "dipole" surrounds personally.
For a screen, I used to think that larger screens were ostentatious, and that anything that's diagonally half of your viewing distance is all you need, but i'm starting to rethink that. The increased resolution and clarity of a good 60" screen is definitely worth it, if the source material is there.
For projection screens, I would recommend going acoustically transparent like the seymour AV. This will allow you to make a false wall behind the screen to place speakers. It gives you a LOT more flexibility with the ever-important center channel.
I don't think rear projection TVs are worth it. Plasma TVs and Front Projectors all the way
