I'm pretty sure that TV.com didn't invent the concept of users rating shows: IMDB had online TV show viewing for a while...
As noted, like most people I typically don't watch shows unless I like them, which means I tend to average in at about a 7. I wouldn't regularly watch a show that averaged out at 5. However, there's some real dogs even among the shows I do like, so it's not like they can't earn a 2-5. Spock's Brain
ain't going to get much of a rating no matter how high you think of Star Trek as a whole.
And of course, if I'm just flipping through the dial while working on something, and watch something for a half hour, I'll rate it for what it is.