They saw the kingdom of God come with power.

9:1 And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power.

This is often a challenge to Christians, put to them by athiests and antagonists who refuse to see and understand what is so plain. They say "Jesus said some of His disciples will live until the kingdom of God comes....how come they are dead and the kingdom hasn't come?

I love how the book of Mark records this account. Jesus said some will see the kingdom of God coming with power and then immediately Mark goes on to describe how 6 days later this prophecy came true:

2 And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them.
3 And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them.
4 And there appeared unto them Elias with Moses: and they were talking with Jesus.
7 And there was a cloud that overshadowed them: and a voice came out of the cloud, saying, This is my beloved Son: hear him.
8 And suddenly, when they had looked round about, they saw no man any more, save Jesus only with themselves.
9 And as they came down from the mountain, he charged them that they should tell no man what things they had seen, till the Son of man were risen from the dead.