Judging from the color pallete and character design, I thought this was gonna be a kids' movie. Imagine my surprise seeing blood gush from a broken nose within the first 8 minutes, imagine the one I got from the guy getting stabbed in the forehead 2 minutes later. Then the... I'm not gonna spoil it but it certainly pushes it over the egde of child friendly. Also "hell" not being substituted with "shell". I kind of assumed Nickelodeon was going loosen the child friendly restrictions for TMNT cartoons sooner or later, I just didn't think it would be this one. Or maybe WB had full creative control over this one, either way I approve.
Speaking of approval, I don't really have any problems with most of the changes from the comic. I don't like that some characters were removed/reduced but otherwise it's fine. The removal of the multiverse aspect is probably for the best, it was such an uninteresting distraction anyway. The implication that the TMNT are part of the DC universe might seem strange but the other DC characters aren't part of it, so it's not really worth thinking about. It's worth it because it means we instead get to see more of Batman and TMNT specific plot devices take a more prominent role.
In general it's a pretty enjoyable movie. Some general issues but I don't expect much from direct-to-video.