Popular interpretations as to what event is anticipated by "the kingdom of God" coming in power include the following:
- The transfiguration
- The resurrection
- The ascension
- The day of Pentecost
- The second coming
A couple things stand out in the passage that are worth noting. First, Jesus has just given indication that the disciples may end up soon forfeiting their lives for his sake. Moreover he links this to his own impending death. Second, Jesus says that "some" will see this event, likely indicating that not all present will experience it. And third, the transfiguration story immediately follows this statement in each of the synoptic accounts. The connection is especially clear in Luke's account where he notes that "eight days after Jesus said this" they went up to the mountain.
Together these indicate that the transfiguration is likely the event anticipated. Unlike the second coming, it was fairly imminent. (And of course if it was the second coming, Jesus seems to be wrong.) The reference to "some" indicates that likely the resurrection, ascension or Pentecost are not in view since essentially all present to hear these words were present at those events as well. Judas is of course an exception, but "some" probably indicates a smaller group still. In contrast, only Peter, John and James were present at the transfiguration. The proximity of the pericopes is probably the most telling.
Outside of the synoptics, probably the most deciding passage comes from 2 Peter 1:16-18:
2 Peter 1:16-18 (NIV) Emphasis added
16For we did not follow cleverly devised stories when we told you about the coming of our Lord Jesus Christ in power, but we were eyewitnesses of his majesty. 17He received honor and glory from God the Father when the voice came to him from the Majestic Glory, saying, “This is my Son, whom I love; with him I am well pleased.” 18We ourselves heard this voice that came from heaven when we were with him on the sacred mountain.
It is evident in these verses that Peter links the "coming of our Lord Jesus Christ in power" with his witness of the Majestic Glory displayed in the transfiguration. If, as is thought, Peter stands behind Mark's gospel and if Peter is the author of this epistle, this provides pretty strong evidence that Mark intends for his readers to link the statements made by Christ with the event that follows.