After Hillenburg and the majority of the original writers left, the series slowly shifted from clever stories involving likable characters to one cheap laugh after another, involving Flanderized characters.
SpongeBob went from the squeaky clean, somewhat naïve, but still clever at times to an annoying pain. Patrick went from a character that merely performed tasks on his own logic to someone incapable of doing anything. Sandy largely went from the likable, clever, but tough squirrel to a major science geek. The only 'Sandy/science' episode from the Hillenburg era that comes to mind is "Sandy's Rocket".
The "Squidward Torture Episodes" are extremely painful to watch; unlike the 'torture' episodes in the Hillenburg era where Squidward (or any character, for that matter) would get punished as a comeuppance, now Squidward (and occasionally other characters) get tortured just for the sake of a cheap joke.
Even greedy Mr. Krabs had a likable quality to him in the Hillenburg era. Now, not so much. And having every other episode be a Krabs/Plankton story is a tad repetitive, especially since a few of these stories were more or less re-workings of previous Krabs/Plankton episodes.
Hillenburg was naturally a lover of the ocean. And his era regularly featured parodies of underwater life. It seems that at first, the writers tried to emulate Hillenburg's sea-parodies ("Shell of a Man"), but after awhile largely ignored the fact that the characters live underwater.
And in some cases, apparently forgot that the characters live underwater and aren't the size of a human being... (0:13)
But the voice cast still enjoys doing the show, which is a positive. And even with unfunny material, one can still sense that Tom Kenny and the gang are having a ball in the recording studio. On another plus side, Hillenburg and some of the original writers are apparently returning for the next SB theatrical film.