I fully agree with regards to the early seasons (1-6) being the best. From Season 12/13 onwards, the show seriously deteriorated in my opinion. I was never a fan of the switching stories, where the three segments of an epsiode were essentially three totally different, barely even connected stories. This seemed to be a running theme throughout the middle seasons and to me, comes across as lazy story telling. It takes skill to tell a story for 22 minutes and I love nothing more when they master this. It also makes the episode far more memorable eg. Remember when Lisa had a substitute teacher who changed her life? Remember when Bart found a comet? Remember when Bart tried to pass his history test? Remember when Homer became an astronaut? Yes!!! Remember that episode where they went to that place, and then somebody did that thing and they all went to that other place and then that person became affected by something and they ended up helping them to do something with 50 lame gags along the way? No, not really.
I find I can watch the early episodes over and over again (with appropriate breaks of course) but the recent episodes, I can barely sit through once. It seems the humour of the early seasons was more intelligence based, whereas today they seem to rely on cheap laughs and predicatable gags. I actually laugh out loud at several moments in the early seasons, but I've gone an entire episode without so much as a smile when watching the later seasons.
Another thing I don't like is space filling. The only example that springs to mind is in Bart's Comet (Season 6, Episode 14), where Homer quite literally sits, twiddling his thumbs. Again, just space filling/lazy story telling. The only blemish in this otherwise all round excellent episode.