It wasn't a sitcom by most people's standards because it didn't follow the formula of:
Quirky Mom
Off-beat Kid(s)
Witty Dad
Oddball Neighbor/Relative closeby
It was different in that all of the characters had their own faults and guises that played into whatever situation they were in.
The humor threw a lot of people off. The laughtrack was not used a lot on the show (I've only heard it three times, I think). A lot of the jokes were very dry, and as a result it lost a lot of the viewers who need their humor BEATEN INTO THEIR HEADS. It's not the type of show that says 'HERE IS THE JOKE! DIDN'T GET IT? HERE IT IS AGAIN! AND AGAIN! AND WE'LL SUMMARIZE!'
The jokes were written more into the conversation.
I hate to say it this way, but you either get the show, or you don't. Chances are, if it's the people who say that it is one of the stupidest and unfunny shows ever, they didn't get it.