JD xNFP there are good arguments for introverted and extroverted, I would guess very tiny eNFP because of his need to be in harmony with almost everyone he can (think of episode where he resolves to learn everyone's name)
Turk ESFP no surprises here
Dr. Cox ENTP....sure, although the weakest of theses seems to be N, I just don't see the abstract and theoretical side in him as I see in JD and the Janitor
Elliot IXFX Elliot is tough, though she seems to be more introverted and definitely a feeler, if you argued S or N or J or P I would probably see your point
Carla EsFJ Carla seems to be more of an S than an N to me, but in both she is very assertive with telling people what they should do. The reason I pick S is she sees problems and how one should react to them, but she doesn't really look at people's potential. It typically is "do this to fix the problem we are having now" as oppose to "do this because it will help you as a person to become better at this skill that I believe you could work on"
Dr. Kelso EnTJ Kelso in almost all situations seems like an S, yet he seems to always know how to deal with problems. He has no problems bending the rules as he needs to and he doesn't really seem to care all that much for traditional things that I think my ESTJ buddy does. I haven't met an ENTJ that I know of for sure...is this accurate?
Janitor INTP He seems way to unusual and creative to be an ISTP, as mentioned in other threads ISTP's are generally the cool guy of the group...I don't think anyone in the show thinks the janitor is cool and he clearly is quite odd. As a very talented inventor and planner he thinks outside the box and has no problems challenging the status quo and making improvements.
Ted ISFP....poor Ted