Immensely entertaining. That said, here are the shortcomings:
1. And this is the biggest one. The fact that a slumdog knows all 20 answers, and all of them by chance, is for one a BIG coincidence. The probability of which happening in real life is extremely slim. But again it is plausible, which leads us to the second point.
2. The language. During the first half an hour the two kids speak in hindi. Which changes into fluent city-bred english when those kids become teenagers. Which in my view is bending backwards to the global audience.
3. It is very bad PR for India. The muslim massacre which mimics the one in gujarat, makes everything visual. Though I am terribly sorry about the incident, showing it to global audience will only add to the stereotype. I understand that this point is debatable, and this is my personal view.
Anyway a must watch.