Well... A standard def. TV runs at 720x480, compared to whatever your monitor runs at(perhaps 1024x768, probably higher...) A TV also uses an interlaced image(basically two images displaying at once) where a computer displays every image. This conversion process drops the quality even more.
Typically, if you run your screen at 640x480, the output is pretty decent. Most text will be readable and all... But of course, many things are made for much larger resolutions.