I got into this show while the first season was in reruns, but before the second season began airing. The show had promise. It was amusing, the characters were fun, cute, charming, and seemed to have dynamic personalities. The plot couldn't be taken seriously, but then it wasn't supposed to be--the show was a comedy after all.
In the second season things started to change. Mainly I felt the writers began taking the show too seriously and trying to play up the more "serious" aspects, while the comedy became more tame and "forced" sounding, causing it to be less funny. Moreover, the characters began to shed their charm and personality in favor of adhering to some classic stereotype, and often the show loved to shove its "obviousness" down your throat. For example, Numbuh Four got the distinction of being "the stupid one." Afterwards, every last line the character spoke seemed meant to showcase his stupidity--even going so far as to misspell four-letter words. Numbuh Three used to like stuffed toys, but became a fan of Rainbow Monkeys in particular, and every episode she was in she had to mention Rainbow Monkeys at least once.
In the third season, things totally hit the dirt. The show became so full of itself that every little idea had to be beat into the viewer's head. Rainbow Monkeys were no longer Numbuh Three's private obsession--everyone except Numbuh Four liked them, and they were a key theme in nearly every episode. Numbuh Four's stupidity got shoved even more down our throats. Characters and ideas you were sick of by this point would recur on a regular basis. The budding romance between Numbuhs Three and Four was made so obvious that it became almost a torture to watch, making me just want to say "Okay, they're in love, we get it already!" Since then, not only have the above problems continued to get worse, but we've also had travesties such as musical episodes, and the show began making regular use of one of the most overused clichés on Cartoon Network: Random, out-of-nowhere child nudity. It went from being promising to being a trite humdrum brainkiller that talks down to its audience. It has lost its value.
