Greatness, as David speaks of it here and as the Bible conceives of it, is godliness. Greatness means being made like our Lord, having his character formed within us. But in addition, greatness includes purpose in life. It is being about the eternal business of God himself, so that the things you say and do count forever. Greatness includes joy, community, being partners with others who are involved in the same things, seeing God's name praised.
Greatness includes wholeness.
I heard the author of the Dilbert comic strip on the radio the other day. Dilbert is a comic strip about life in a modern technological firm, and everything that takes place is a testimony to how foolish, self-consumed, competitive, and ridiculous such environments often are. But this psalm says that God stoops down to make us great, not ridiculous. He lets us be about things that are really important. We can have real purpose.