its to do with source material, connection type, picture settings and screen ability
source material: the quality of the program you are watching is one of the big factors, disney dvd's are usually the best quality reproductions availalbe. many are THX certified complete with a thx optimiser program on the disc.
quality goes from excellent (7hd loop) to good (disney dvd) to poor (foxtel) even a pgood analogue signal will surpass foxtel in quality.
Connection type: use atleast s-video when connecting sources but where possible use component or rgb/s. with analogue video connections, the more seperating of the signals the clearer the image.
component is the most common of the high quality connections, appering on many digital decoders, dvd's, PC's.
there is rgb/hv using the VGA connection but this is rare on anything but high def tuners etc.
Picture settings: configuring your contrast/brightess/sharpness etc settings to the right levels can make all the difference. get a THX dvd like disney's the lion king and use the optomiser.
contrast and sharpness are the big ones. turn off all image inhancers in the menu, all colour brightners, edge enhancers, black levels etc. try and make the image as natural as possible, many displays are set to look bright and colourfull for display purposes so they stand out in a store, however, these settings are unrealistic for general use and can distort the images naturalness to the point of annoyance.
screen ability: the scaler and panel driver are usually to blame when a usually good source looks crap. digital panels, especially high res units have a hard time when 1:1 or 1:2 scaling can not be achieved. the image usually suffers from double ups of sharp edges or missing eliments. either use a native res between devices or use an external scaler such as the one in the panasonic hdtv tuners.
the panel driver can make or break the screen. a good driver will allow good colour representation (8bit/pixel or 16.7mil colours) a poor driver usually falls to 6bit or 252thou colours. with plasma panels using PWM to shade (strobing the pixel instead of dimming) the speed of the driver usually is a limitation.