Nothing can be more unsettling than waiting for what we expect. And when we don't know exactly what it is that has been promised, the waiting can become like "what's the point?" The 120 were waiting, not knowing what or how it was going to happen. However, the issue here was that they were there in obedience to Jesus' command to remain. So many times, with God, the "go, walk, fulfill" part of His promise is found the "stay" part of it.