When Notch first talked about achievements months before it was released, he said that he was not going to do any sort of grinding achievements. None of the "Kill x amount of x" or "collect x amount of x" will ever be implemented. Because he himself does not like them.
The Meaning of Life, the Universe, and Everything.
In a large pokemon forest
Ya, killing pigs hours end is not a kind of achement that is fun. Because you would have to grind to get a achemement, and serously, who like to grind to get a achevement that is not worth anything and that wont do anything. It might give you a bit of pride, but so what, you just wasted time hunting down pigs hours end...