The difference is the original TMNT cartoon is your typical 80's show which had slapstick humor and bizarre sci-fi plots self-contained in each episode. Shredder and Krang would come up with a new scheme to take over the world every week and the Turtles would always stop them by the end of the episode. There were also a lot of other recurring villains to keep this from getting stale, as well as one-shot villains, and most of the plots are pulled out of 1950 sci-fi movies. The reason the original cartoon became so popular is because for an 80's show, the main cast and villains were incredibly well-defined with likeable personalities. The show had a, "This plot is so stupid its actually funny," kind of mentality to it. The show made fun of itself and never took itself too seriously, they break the 4th wall in many episodes. To date, Krang, Bebop and Rocksteady are three of the most popular and well-liked TMNT villains of all-time, and they all originated in the old show. Granted I may be bias because I grew up with it and thus have nostalgia for it, but just don't go into the old toon expecting TMNT 2k3 or what cartoons are like today.