I find looking slightly downwards far more relaxing than looking upwards. I believe it is also a medical fact (the latter hinders blood flow). I have actually had my smaller MXs placed on the floor in the past; it was nice to watch and also looks great as long as the surroundings are mostly empty.
Now that much of the content we watch is HD and you can thus sit closer to the screen, the top edge of a 55" TV can actually end up really high if the TV is placed on as high a stand than the former smaller TVs were.