1. The protagonist (I'm just going to say "hero" from here on out; it's shorter) is not an adult (and usually male).
2. The hero is an orphan (usually both parents are dead, although there is sometime one (usually the mother)).
3. The hero is "special" in some way.
4. There is a prophecy, generally related to the hero.
5. There is an old mentor of some sort, usually a male. (We recognize this character as "the wizard.")
6. There is a quest of some sort involved that only the hero can complete.
7. There is some kind of descent
8. The hero has companions who help him on his journey.
9. There is some sort of "dark lord" who can only be defeated by the hero.
10. There's an absence of technology.
The thing of interest to me with this new movie is the protagonist. Yes, protagonist. Historically (I use that term a little loosely, but the Turtles have been around for three decades, now, so there is some history involved), the protagonists of the Turtles stories have been the Turtles themselves. If you look at our list, the Turtles fit virtually all 10 of the points, even the one with the prophecy at various times in the stories.
But the Turtles are not the protagonists of the movie. No, that honor goes to April O'Neil. In the comics and other, previous stories, April is merely one of the companions, but she is the main character in the movie, and the Turtles are the companions. How can we tell?
Well, let's look at the facts:
In this movie, April is an orphan. [I don't remember this being a part of her story previously, but, then, maybe I'm just not remembering it. Actually, I don't remember it ever coming up before, which would lead me to think she had parents.] Effectively, she's also not an adult. Technically, she's an adult, but she's being portrayed as someone not quite there yet, basically, coming of age. April is the special one and, loosely, of prophecy in that she, in this movie, was the owner of the Turtles back when they were just turtles. She was their savior.
This thing where they made the Turtles her childhood pets is a huge change and plunges April into the role as the story's protagonist. The Turtles become her companions on her journey to "destroy the dark lord," in this case Eric Sacks. The goal of the Turtles is to keep April alive and assist in her journey. Splinter still serves as the mentor figure, and the world of the Turtles, into which April descends, lacks technology. Sort of. There is technology, but it's Donatello's "magic."
I find the changes they made to the Turtles mythos very interesting, probably because it comes in the midst of this fantasy series, but, still, I find it interesting. I have to assume they made April the protagonist because she's easier for audiences to connect with. After all, they have very much downplayed the teenager-ness of the Turtles in this iteration, so there's not that for the audience to connect with. I also find it very interesting that in making the changes they made, they brought it into alignment with the fantasy paradigm I've been discussing. It's not surprising, but it's interesting. Of course, super hero stories tend to follow the fantasy model, even if April herself is not the super hero.