I know one issue is the fixed resolution on TVs. Say you have a 50" 1080P LCD TV and resolution maxes out at 1,920 x 1,080 and you have a 24" monitor that maxes out at the same resolution. Graphics are going to look nicer on the monitor because the pixels will be smaller on the monitor compared to the TV.

Response times could be another issue but I'm not familiar enough with that to talk intelligently about it.