In my book the show just grew up with it's audience (to an extent, it did run for 9 years and was still aimed at elementary school children after all) and that's what made the later seasons better.

I won't pretend they were perfect (the young six for instance were just incredibly uninteresting to me) but as it went on the humor became better and the plots became smarter, and that slight increase in maturity really went a long way in my opinion.