I love this show for so many reasons.
The first and foremost is the show's evolution tied to my generation, much like the Toy Story franchise. When the show started, it was an innocent Saturday morning cartoon with lots of cute characters and puns. I was in grammar school.
By middle school, the show had gotten significantly more advanced. Plots went for entire seasons, and characters were killed off. Bigger threats were alluded to.
The last season premiered when I was in high school, and everything got a lot more mature and darker. The bigger stuff alluded to in the previous seasons was revealed, and they were not whistling dixie. The situations were really dire, and the characters showed how desperate it was. They had internal and external struggles. The characters had grown up and evolved with the viewer.
I respect that. It's something other shows that tried to stand the test of time (TMNT, anyone) forget: your audience is constantly maturing.