The Turtles would not have been so ridiculously obsessed with pizza, and the Shredder would not have had as one of his businesses a restaurant called ‘Ninja Pizza.’
April would not have been a reporter and constantly need to be rescued by the Turtles.
The Shredder would have been seriously malevolent.
Among other things, there would likely have been no moronic henchmen like Bebop and Rocksteady.