What is a memorial for?
From the Spring 2002 City Journal:

Whatever monument we finally choose, it should rise in a square amid a rebuilt center of business, not in the midst of a 16-acre necropolis. Even though emotions are raw, we have to keep in mind that we are building for the ages. Fifty years from now, the best memorial to those who died in the attack will be that their monument adorns what is still the world trade center.

I'm sold. This is it.

(The essay is also available at WSJ Opinion Journal, but City Journal has a better image of the design. See also the full redevelopment plan for the WTC site from the Autumn 2001 issue.)