don't be too upset with Northstar's quip
If I could be as amused at 50 as I was at 25, then something must be right. I've always felt that cartoons tended to be undervalued by adults, especially children's cartoons. But there is often surprisingly a high degree of thought put into making them. Nickelodeon in particular is pretty high class stuff.
Aaahh!!! Real Monsters is probably one of my favourites out of Nickelodeon. I watched some episodes by DVD 2 years ago and was astonished by how much extra you get out of it being an adult, including some references that would have whizzed by if you were a kid. The series deals a lot with trying to fit in, being different, conquering anxiety and the value of friendship. These are good things for cartoons to show though kids might not fully grasp the big picture.
I also ended up going back over Thomas the Tank Engine and Friends (particularly first 5 seasons). The episodes are short (5 minutes each) but the level of detail and the storytelling is pretty interesting looking at it from an adult's perspective. At the time, they created actual model railways and trains to remotely control and film that. It does give a feeling of realism and I definitely felt that as a kid. They replaced that with CGI in the 00s but it's very cool seeing the original sets. The stories are actually pretty fun as well and not without some comic irony, though some adults think the engines are a bit too naughty, bully each other and therefore teach bad things to their kids! (when the point is that engines will misbehave and make mistakes, but they can always make up for it if given second chances and be useful again, and it's pretty much expected for a reverend to write it this way). There's something amazing about the series that they continue to write for it even now.
Season 4, one of my favourites from a storytelling perspective: