What you may be seeing is a standard definition picture on a HD panel, which does not look very pleasing.
In the meantime, you may want to either see if you can bump up the output resolution on your DirecTV box, or as a last resort, you can watch local HD channels with an antenna.
I just bought an LN40A500 HD TV. I have ordered new DTV HD service with new box, but, will not have everything rewired and set up for a few weeks. I tried to run the component wires from the old DTV box to the new TV, but, the picture was awful. Is this a just bad idea or is there a setup issue I am missing.