the sound system. Sound is just as important as video quality. If you have lousy sound, it won't matter how great your picture is.
If the sound is set up correctly, it's easy on the ears and easy on the brain to process. Movies will be relaxing to watch (on the brain) even at high volumes. It's not the volume that is key, but the overall sound setup. Bigger isn't necessarily better. Speaker placement, center channel quality, a good sub and a decent AV receiver will make a huge difference in the overall experience.