30 Rock uses racist jokes in order to satirize, whereas Big Bang just goes for the easy racist dig in hopes that people will laughs.
In 30 Rock, there are nuances to the supposed blatant racism that the viewer is expected to recognize. For example, the show makes fun of Liz’s subtle racism (pronouncing an Indian page’s name as Ah-mahnd-ah instead of Amanda or assuming Tracy Jordan can’t read) in order to point out that people can sometimes be racist, even if they consider themselves to be free of bigotry. In another episode, Let’s Stay Together, they satirize NBC’s diversity program, and in doing so, point out its limitations. The jokes may be racist, but they are calculatedly used to draw attention to important issues.
Big Bang, however, completely lacks the social commentary aspect. The digs are just for the sake of a cheap laugh, insofar as Sheldon will make incredibly racist jokes for absolutely no purpose. People may argue that the purpose of these jokes is to demonstrate Sheldon’s social obliviousness, but there are plenty more ways to point out that he is socially oblivious— methods that do not require racially charged jokes to the extreme frequency that he uses them. Furthermore, Sheldon is never punished for his racist digs, which suggests that these are jokes that can be condoned. Conversely, 30 Rock characters are almost always punished, even if it comes in the form of public embarrassment. Characters on The Big Bang Theory are blatantly offensive because the lowest common denominator* of people will laugh at it, not to point out racial issues or for character development.
30 Rock has been nominated by NAACP on four separate occasions, precisely because they are not afraid to confront complex issues of race head-on through the medium of satire.
If you want to read more, here is an article on identity politics and 30 Rock.
*I am not suggesting that if you watch The Big Bang Theory, you are always of the lowest common denominator. There are plenty of people who recognize the racist elements of the show and watch anyway, just because they need some light comedy to wind down with after a long day.
(rebloggable by request)