Like many of Miyazake's movies, Ponyo is first and foremost visually lovable. Cute creatures moving in wonderfully strange yet lifelike ways - what's not to like? Secondly, the English language versions are regularly dubbed by famous actors, and there's just something great about hearing those familiar voices as cartoon characters - especially Liam Neeson and Tina Fey.
The movie is pretty lighthearted and jolly, but there's not much to the plot. It's got some elements of The Little Mermaid (the Hans Christian Anderson version, not the Disney one), and a bit of eco-moralizing, but overall it's pretty random. Towards the end it suddenly seems to be about the fate of the universe, but it's never made clear how or why. The main characters are confronted with some kind of dramatic trial, but you don't know what they have to do or what the stakes are really, and you kind of figure it'll work out ok anyhow, so who cares. Fair enough for a children's movie, but it's not nearly so compelling for adults as other Miyazake films.