Well, seeing as I'm a pretty big fan of him, here's my opinion:
Most of the reasons for Pterano's appeal have been listed already. His possible insecurity, his ability to think more logically than most of the grownups. But I think another reason is his motivation: He does want to rule over the Great Valley, but he appears to want to do so as a loving tyrant. He wants to use the Stone to create a paradise, as he says, as a sort of perfect world, with hm ruling over it. He does not necessarily have an evil plan, but he is willing to go to evil lengths to get it.
But another reason, which nobody has touched on, is simply that he is voice by Michael York. York is a very well known actor, very respected. He's done Shakespeare films, The 3 Musketeers, all sorts of films before. But I thin this is his first voice-acting role. He's alo the first major actor to star in a Land Before Time film. Granted, they have gotten famous voice actors before, like Rob Paulson, Frank Welker, Jeff Bennet, etc. But York was the first major Hollywood actor. And naturally, ifa person is already a famous actor in Live Actin, you can almost certainly bet that asvoice-actor he will be brilliantly hammy (think Tim Curry). And of course, York hams up the role, chewing the proverbial scenery, but in a brillant, flamboyant way.