The games. They not only have games with incredibly more depth than tablets, but they also have better capabilities for having better games.
I recently bought my very first iphone a few weeks ago, and I was actually excited to start downloading games and playing on it. But then I found out that there's not even a game like ALTTP on it, and that all of them are very shallow experiences and offer nothing great for real gaming. I looked at numerous top 50 app lists and recommendations online and I've come to the conclusion that the games I enjoy playing simply aren't there.
That's why I love my 3DS.