On the one hand, my reaction to the Turtle design is a resounding "Meh, who cares?" It's not as if the 80s interpretation of the comics was all that much less broad in terms of each character being a one-note character. So, have they REALLY trod that heavily upon the original design? And in the original Eastman and Laird comics, the turtles weren't even that distinguished. Their main difference was in the weapons they wielded and who led the team (Leo). But the rest? Not really all that distinct as I recall.
But I think I also get the deep sense of loathing that others have for this. There is something decidedly "lowest common denominator," which is all too common in stuff with Bay's name on it. They go for the most basic, over-the-top, and dumbest approach to things, and the turtles' designs strongly suggest that this same approach is being taken with this film. I'm not saying it needs to be high art, but the film also doesn't need to treat it's audience like a bunch of morons, and let's face it: that's EXACTLY what Michael Bay films do. They assume the average audience member is lucky to have two brain cells to rub together, and structure the film accordingly. More than just being a fun ride that doesn't take itself seriously, there's a tone to Bay's movies that seems aggressively condescending. Like "I'm gonna make the dumbest piece of **** I can, and you're gonna love it because I put 'splosions in it, you ****ing idiots. Now shut up and give me your money that you are clearly too stupid to be allowed to keep." To me, that's the vibe I get from Bay's movies.
Plenty of other action films and action directors and producers make over-the-top movies. The original Robocop, for example, is RIDICULOUSLY over-the-top, but it is so as a parody of action films that manages to simultaneously be exciting. The first Pirates of the Caribbean didn't take itself seriously, either, but it managed to be a rollicking good time that seemed to treat the audience as if it was in on the joke and part of the fun, too. But Bays' films just have this particularly stupid quality to them, and I can see where people would get that same sense from the design of the turtles. If you're a fan of the originals, I can see where this would particularly irritate you. But even if you aren't, I can see where this ends up being just yet another crapfest from Michael Bay that you've had about enough of. It's not even a question of "ruining childhoods," as much as just making crappy movies that waste gajillions of dollars.