I must applaud the writers for making a show without a ton of mutants

[Coola Yagami]

The characters did have longer arcs, but it was still kinda 'freak-of-the-week'.
Let's see in season 1 alone they faced Purple Dragons, Mousers, Nano, Invisible Ninjas, Garbageman, Shredder, that villain Mikey faced as Turtle Titan, the Monsters from Underground, Kirby's monsters, random mobsters, and Utrons, with a cliffhanger that ends with them being sent to outer space.

The main difference is that this series has 2 or even 3 part stories, hell, even 3-part filler stories, so it does feel more fleshed out, less episodic and not exactly freak of the week.

However, season 1 did have episodes where the Turtles just faced Chris and Xever again without introducing new characters or villains. I assume we'll be seeing more of Chris, Xever, Karai and Shredder again in these episodes too. Mutagen Man himself doesn't count as new since he's been around for a while.