According to the commentary for the first season DVDs (I forget which episode this was mentioned in) Matt Groening wanted the cartoon to have a true-to-life/live action feel to it. In other words, he wanted a cartoon to act as un-cartoony as possible, and therefore not include any crappy slapstick nonsensical stuff in it.
I don't know what the hell happened, but the Simpsons turned into crappy slapstick nonsensical stuff. Exactly what Matt didn't want the show to turn into.
What really pisses me off is that they should have made a movie when the show was good. There are rumors that they will make a movie when the series ends, but I bet it will be horrible. If they had made the movie while the Simpsons was still the great thing it once was, it would probably be one of the greatest ever made (at least in my opinion). When (or if) they make a movie in two or three years, it will be the biggest pile of garbage in Simpsons history. I'm guessing they'll cram more pointless celebrity appearances in 90 minutes than no one can ever even picture within the confines of their vast imagination.
I hate the Simpsons so much now.