I find the heavy Squidward abuse and overly gross factors in some episodes were the main cause of this show going downhill, at least to me. I do think it is getting better though. Season seven hasn't had any severely gross episodes yet which is definitely a GOOD thing, and no overly torturous Squidward episodes with the exception of Cephalopod Lodge in my book. But yeah I guess the other main reason was because the show got too childish and too many annoying potty jokes and all that crap, but I only think the show was going downhill in season five due to so many disgusting episodes and/or episodes with pathetic plots, but now sometime in season six the show started to climb back up again with some great episodes in my book. I really loved The Slumber Party, Ditchin', and Porous Pockets!
Here's my scores for all of the seasons:
Season One: 8.7
Season Two: 9.8
Season Three: 9.8
Season Four: 7.5
Season Five: 3.5
Season Six: 7.8
Season Seven: 8.5 *And this is for at how it is going right now. Not the whole season entirely because it hasn't ended yet.*