First appearance of the Technodrome's portal chamber and Krang and Shredder's dimensional portal generator.
First appearance of the completed Turtle Van.
First appearance of General Traag, was well as the Neutrinos.
Although it's never stated on the show, the gray stone warrior's name is Lt. Granitor according to the instruction booklet of the NES game Teenage Mutant Ninja Turtles: The Arcade Game (1989).
The Nutrinos would later return in the episodes.