I think it comes down to this: professional TV critics tend to know very well how TV shows are produced, so they understand clearly the self-referential humor of the show, which was created by Tina Fey (a former cast member on NBC's Saturday Night Live). Problem is, most of the TV viewing audience have no idea how TV shows are produced, so they don't understand the premise of the show; that's why 30 Rock has such poor ratings.
30 Rock has one or two really funny moments each time, but the Tracy Lawrence character and the Kenneth character are horrid, completely unfunny...I fast forward through the show every time they come on.
I do enjoy them making fun of the TV biz, though, it’s some really good satire.
Ed