The Observer, Rowan Moore
It's easier to say what the "mosque at Ground Zero" is not, than what it is. It's not a mosque, and it's not at Ground Zero – only nearby. It's not a "clubhouse for terrorists", as some objectors have called it, nor a work of "triumphalist stealth jihad". It does not "loom" over the "hallowed ground" of the 9/11 attacks, which cannot be seen from its site.
As to what it is, the explanation is not at first very enlightening. Park51, to use the project's proper name, is "a friendly and accessible platform" that "enriches lower Manhattan in body and spirit, with ecologically conscious design and operation". However, its architect, Michel Abboud, makes things clearer: it is a Muslim version of the YMCA, or the many Jewish community centres in New York. That is, it will have a swimming pool, basketball court, childcare and exhibition facilities, a library, auditorium, restaurant and catering school. As with the Christian and Jewish versions, you won't have to be a believer to use these facilities.