You see, what separates Hey Arnold! from most cartoons, is it's characters. Usually there are two to four main characters and maybe five to eight side characters, none of which get a whole lot of attention. In Hey Arnold!, you had an entire neighborhood of characters each with at least one episode that showed you who they were. They're background, personality, struggles, and overall triumph. Where else do you see that? And Arnold was such a sweetie, you couldn't not like him. This show deserves far more attention than it gets, that's for sure.
Also, Helga's huge secret crush on Arnold was the funniest thing ever.