For me it needs to be:
Watchable for adults too - Hey Duggee does this well, some shows are just tedious or unwatchable after a few episodes
Informative, tell kids about the world around them. I think Octonauts does this very well with the emphasis on a new place or creature each episode. I'd like to see more documentary style programing suitable for young kids.
Age appropriate. My kids are 5 and 3, I don't want them being shown adverts or suggestions for tv shows which are suitable for 10 year olds
Ethical without being over the top. Yes, tell my kids about why it's important to care for the environment. Yes, tell my kids that different people have different bodies/families/beliefs, and that this is normal, and what they will encounter in the wider world.
Encourage critical thinking and making their own decisions about what they choose to believe in. Encourage wider discussion, and for children to ask questions.
Not just a thinly disguised method for selling toys to children - Paw Patrol I'm looking at you here.
It should be possible to toggle on/off Autoplay the next episode. It should also be possible to toggle on/off the 'are you still watching?' prompt. Nothing worse than being elbow deep in the washing up bowl and hearing 'mommy it stopped!' from the living room.
It should tell good stories, and be high quality. My kids love the Julia Donaldson adaptations, especially the Gruffalo, and will watch them repeatedly. I enjoy them too, because the stories are lovely and the animation is beautiful. I loath Cocomelon for the same reason as a PP, the characters are wierd looking, the animation is jarring and the songs are repetitive and dull. There is a lot of that kind of dross on Netflix.
I think those stories should come from all over the world too, tell stories from other cultures, as well as having a good range of British culture. Less American accents. Again, Octonauts does this well.