I didn't think it would be very good when I first got wind there was a tv show. I initially started watching it because of Holly. I guess I was preconditioned to think that cartoons based on videogames always sucked. But here it is, 5 years or so later, and Pokemon is still popular on television, and I'm hopelessly addicted.
In a word, Ash. Sure, my favorite character isn't Ash, and my daughter's favorite character isn't Ash, and in fact I don't know anyone who's favorite character is Ash. But, like Goku to DBZ, Ash holds the show together.
Unlike Goku to DBZ, Ash makes the show because he is Everyman. He's not the smartest, he's not the bravest, he's not the most talented, and he's not the strongest. What Ash has that everyone can relate to is loyalty and determination. He's just a regular kid, like the kids watching, yet he goes far just because he's determined to make it. He has to work hard, and make tough choices, just like everyone else.
As the show goes on, he does improve his skill, but only with the help of his human friends. His loyalty to them and to his Pokemon is tested on a regular basis, and he passes with flying colors. His decisions aren't always the right ones, but he proves time and time again that his heart is in the right place, which is 2 inches below the breast, which he doesn't have because no men in Pokemon have nipples...
So, the hero is just like us, but his world has cute creatures and exciting situations. Therein lies the escape from reality, and the reason we are so interested in his adventures.