Maybe it's because the audience is Waaaay too enthusiastic now. They laugh at anything, kind of like those cheesy comedy clubs that tape the comedians after the audience is drunk.
Maybe it's because the actors on the show (with the exclusion of the woman who plays Sally, who I think the world of, but that's another review) are oh-so full of themselves now. They are a hit, and unfortunately, they know it. They're preening; hamming it up. The subtlety is gone from their performances.
I mean, I love John Lithgow. I think he's incredibly gifted. And in fact, I don't think he's the one ruining the show. He's just following orders, I think. Because watching him, I can see that he's still good. But the writing falls short, and his co-stars (except for Sally) are so busy trying to be just like him that they all end up looking foolish.
For example (I know you're waiting for one): They've started doing lots of sex jokes. That's usually, but not always, the first sign that the writing is going downhill. Another thing they're reaaaally relying on is the physical comedy, which in the first season was fantastic. Harry and Sally learning how to dance was a great scene. When Dick first learned how to kiss and mistakenly thought each kiss was followed by a slap across the face -- positively inspired!
Now, well, someone has mistaken physical comedy for funny. Not true. The material behind it, not just the pratfall, is what makes it funny. And the material is starting to get stupid.
I'm sad. Some shows manage to handle semi-instant popularity, such as Seinfeld and... well, Seinfeld. And some shows get cocky. I'm sorry to see Third Rock go.