I saw this post too late and probably no one will read but anyway:
Having watch every episode like 20, 30 times each, the thing that makes it so good for me(apart from all the things mentionted in this thread) is the realism. Not the realism of the situations the characters get involved, (cause actually along the seasons passes the situations are more exagerated) but the realism of the details, the simple things. The most noticable are the conversations. There is no other show on tv that replicates social interaction as well as seinfeld does. What in my country (argentina) is called as " hablar al pedo". Talking about "nothing". When you are with your friends, you just talk, talk without making much sense, most of the time talking without being totally serious, talking just to hang out and have a good time. Thats why is so relatable not only for americans, because it does an excellent job in portraying (?, didn't find the wright word) how people behaves in general ( how they sit when having dinner, their face when waiting, how they react to something,etc). Every little detail that determine how we act.
A good example to ilustrate my point is the episode "The dinner Party" . Episode where elaine and jerry go buy the babka, and george and karamer the wine. Just watch it and take a closer look on how they act in those situation, and imagine yourself in that situation. My favourite part of this epiosde is when kramer and geroge are waiting for the car doubleparking and this: EORGE: I really can't comprehend how stupid people can be sometimes. Can you comprehend it?
KRAMER: No, no I can't comprehend it? GEORGE: I mean we can put a man on the moon but we're basically still very stupid. The guy who's car this is? He could be one of the guys who built the rocket. You see what I'm saying?
KRAMER: Yeah, he could build the rocket but he's still stupid for double-parking and blocking somebody in.
GEORGE: So you really understand my point about building a rocket and double-parking.
KRAMER: Yeah, on one hand he's smart with rockets and on the other part he's dumb with parking. . . . It's cold out here huh?
This is just exactly a conversation i have with a friend, different topic of course, but the same "style". My friend actually said " do you understand my point on ..." just as george.
Well i end up with a wall text, so TLDR: I love how larry david understand and show how we behave, how we act, how we talk.