HD gaming is not so HD?

I've bought a microsoft's hdmi cable for my xbox360 so i could play games in HD. I have a Panasonic plasma, and it's HD ready and it's supposed to give '' awesome gaming experience'' etc. But the problem is, games don't seem so HD to me. Actually, they seem the same as they used to be before, before the hdmi cable. I watch gameplays and walkthroughs of other players on youtube, and graphics are stunning. in comparisson, mine are shitty. Can someone help? Maybe I didn't do some settings on my TV or sth? thanks in advance