The original cartoon was aimed at kids but the comedy was aimed at older viewers and they still pushed it as hard as they could.
Considering how strict the censors were back in the 80's and early 90's, I'm surprised the original TMNT cartoon got away with as much as it did. They was still a lot of contact during the fight scenes, (unlike in stuff like Superfriends), and all the characters played off each other well. As silly as Shredder/Krang got at times, it was still a classic story of good vs. evil.
The great thing about the original show is all the main characters had very distinct personalities. The Turtles, Splinter, April and the news crew, Shredder/Krang/Bebop/Rocksteady...all had chemistry and great character interactions amongst each other and other characters. No character was a bland re-tread of anyone else.
I actually watched through Seasons 1-8 of the original TMNT cartoon on DVD, and aside from some dull/silly episodes here and there, the show was very good for its time. It really reminds me of why the series was so popular when it came out, there was really nothing quite like it.
As for what Nick will do, I hope its the original cartoon characters and humor but with the 2k3 series continuity and fight scenes. That would work for me.