for one, it has a unique setting for a kids show. instead of being based on a real world, it actually resembles a city. there's garbage and grime. the buildings have been worn down, and it the world feels geniunely lived in. and it's colored with these great watercolor backgrounds.
the music from
jim lang is also fantastic. usually jazzy and laid back, but it can also be pretty emotional. it's unlike anything i've heard on the channel before or since. it lends a great style to the show.
of course there's the vocal talent. i don't want to single any one person out since all the kids do a great job, but francesca smith is particularly good as helga. the first and second arnolds on the show are also really good at what they do.
i think what stands out the most though is that it doesn't talk down to its audience. not every ending is a happy one and if it is, there's often a price. in arnold's christmas, helga is left with nothing but the satisfaction of getting arnold to believe in miracles again. it comes at the cost of her material gifts
and wanting his affection in return for getting him a great gift. the concept of giving someone a
belief for christmas, of indulging an idealist, i don't think i've seen explored in cartoons or television in general until abed's uncontrollable christmas over a decade later.
there are a lot of examples of the protagonists not getting what they want. not totally. it lends to a realistic depiction of childhood. there's also a lot of different family types and they're never pointed out as being different. arnold is raised by his grandparents, helga has a verbally abusive dad and a negligent alcoholic mother, stinky is raised by his single dad, phoebe's parents are an interracial couple, and patti's parents are little people. no one really questions it within the show, until arnold asks about what happens to his own parents.