I think it's almost impossible for any sitcom to go on for more than 2-3 seasons without running low on ideas and spreading the plot too thinly. I don't know what it is, but they just loose something.

I do watch rather a lot of sitcoms (or have in the past) but almost without exception they are shallow and can be watched with half a brain cell switched on. If you're at all awake while watching sitcoms (like many formulated movies, usually action) you can tell how it'll end before it's started.

I watch pretty much all varieties of film and tv programme, but i guess you could have a ranking scale of how likely the programme is to be absolute pish based on what genre it is without knowing anything else about the show. Reality TV and sitcoms would be near the bottom and work your way up to documentaries and big-budget dramas (although there are plenty of rotten documentaries and dramas out there, and a scarce few good reality TV concepts :P)