Funny characters, entertaining songs, engages the kids and inspires them to use their imaginations and/or make their own puppets, etc.
Season 1 is the best; Season 2 had some great ones but also questionable episodes with lessons that weren't really the greatest and they got rid of the "Oobi's Game" parts at the end that helped with memory/matching. But the first season is fantastic and absolutely worth watching or showing to your kids. My daughter adores Uma and often pretends to be her with her hand.
Edit: after seeing other parents' reviews, it appears as though people are angry that there are gender stereotypes?? I don't really see a problem with the girl having a flower and the male best friend (Kako) having a hat. This is simple stuff. It hasn't harmed my toddler or discouraged her from liking trucks and trains and cars. We don't need a million shows with genderfluid trans characters, it's okay to have a show with an obvious girl and an obvious boy (who are HANDS mind you). Sheesh.