I disagree with everyone!
The room is the most important component in a home theater. Without you wouldn't be able to use any of the other components listed. And the room has the biggest impact on the performance of every other answer listed.
TV - no light control makes it harder to see, the room size dictates the screen size, how far you sit from the screen, how many other people are able to enjoy you system at the same time, where your TV is positioned (i.e. windows, fireplaces, hallway, doors, etc.)
Speakers - many of the same things as above. Room size will dictate speaker size, type, location (think WAF), and most importantly -HOW the speakers sound!!
Receiver - room can dictate placement (ability to run cables to other components), how powerful (bigger room, more power), how many channels you need (is it feasable to get a 7.1 system?), potentially lifespan (crammed in a closet with no heat disapation).
Source (DVD, Xbox, Sat) - room dictates location (where is the cable jack located)
Ultaimately with a poorly designed room, everything else becomes harder. Thats why we all try and find the perfect room (or design and build it like I did) for our home theater. It is also why the market is shaped as it is - room treatments, brighter and smaller projectors, in-wall speakers, flat panal TVs, wireless (not so much, but it exists).
So I know this is a loaded answer, but many people think bigger, better, MORE expensive equipment equates to better performance. While, in reality, with a good room, modest equipment will blow away expensive equipement in a poor room.