The original comic was darker, and much more violent than the cartoon was. Even the first movie to an extent.
What truly bugs me is that we will obviously never see a TRUE TMNT cartoon, like the Maxx or Spawn that's made entirely based off the comics, and much more adult and deep. 2K3 was the closest we came, but it went off on its own tangents. I will say, I think you might just like Fast Forward. It was MUCH lighter than the rest of the 2K3 series... unfortunately, a little TOO light in some cases. And the characters' personalities were watered down... Splinter's once mentioned habit of watching soap operas became a complete obsession in that series, angsty Raphael turned into Oscar the Grouch, and Mikey was an idiot.
I've said this about the Smurfs, the cartoon is far more recognized than the rest of the legacy... and that's not necessarily a good thing. As far as Turtles go, I really hated it when merchandisers and other adults gave everyone Mikey's at the moment 80's surfer dude personality, and turned their association with pizza into an obsession (in the comics, they actually preferred Chinese take out!) Above all, the cartoon seems to make the notion of mutated turtles that live in the sewer sillier than it sounds. I mean, the idea of an alien humanoid flying around in spandex, and a rich guy with emotional baggage dressing up as a bat are pretty silly too. A certain giant rock creature seems goofy too, but he's one of the most tortured characters in comic history.
As for darker and violent... well, again, the comics these things are often based off of are a lot more depraved. But it was something that came about around Batman TAS... it was possible to do grittier stories and have characters actually hitting each other (Challenge of the Superfriends has Superman catching villains in their own traps... then letting them go... talk about irresponsible on Supe's part). Without Batman TAS, we wouldn't have had the X-Men cartoon. And the key to longevity with super hero cartoons is that they can be rebooted and re-imagined to an extent. Like how many different writers shaped the characters over the years.
I will say this, though... Archie did a MUCH better job with most of the other mutants. Leatherhead being a poverty stricken human that stole a mystic object, and was cursed to be a mutant gator... then being used by Shredder under the lie that they worked for that sorceress. Wingnut and Screwloose coming out of no where and destroying skylights to try to find the same villain that was pestering the Turtles, Krang. And then spinnning those guys off into another series? Seriously, I hope whoever does this cartoon read a LOT of Archie turtle comics.