One of my favorites growing up, Hey Arnold was about a boy (named Arnold, surly) that's basically Mr. Goody-Two Shoes. He's surprisingly well-rounded for a nine-year-old, and kids his age (and sometimes, even adults) come to him for advice. There's also a sub-plot (my favorite portion of the show) about a girl named Helga, the school bully. She especially likes to victimize Arnold, but that just covers up her feelings about him; she's madly in love. It is sort of creepy to see the extent that Helga will go to, such as making shrines of Arnold out of garbage, sneaking into his house, or even stealing things from him, such as his hat or gum.
CHARACTERS: Such a diverse set of characters makes this show look like the "watered-down" version of The Simpsons. And they are a little connected, considering that the creator of Hey Arnold's borther-in-law is Matt Groening, and the voice of Homer doing a handful of characters on here. Sure, there are some stereotypes, but most of them bring something new up to the table.
There ARE some characters that I hate from Hey Arnold, though. Rhonda is nothing but a whiny rich girl who seems to never learn her lesson, and Lila is annoying.
ANIMATION: Nothing wrong with it. Fine by my standards.
HUMOR: Love it!
Overall, a great classic.