Mike Scully was executive producer starting with season 9. From Wiki: "Scully has earned an unpopular reputation with some Simpsons fans who feel the show's quality began to decline during his tenure." He wrote an episode about Burns wanting to turn a bunch of puppies into a vest back in the sixth season----and it all falls into place now.
http://en.wikipedia.org/wiki/Mike_Scully
Al Jean picked up with Season 13, but by then I've lost interest in the show. I have noticed recent episodes are funny, but it lacked that charm previously mentioned that I perceived.
http://en.wikipedia.org/wiki/Al_Jean
I also don't care for his anti-Family Guy stance... oddly enough, he's not alone. Some of the criticism is either jealousy or hypocrisy, and they end up insulting FG viewers, who also watch (or once watched when their show was good or still on the air) their show.
http://en.wikipedia.org/wiki/Criticism_of_Family_Guy