On music charts, the most impressive hip-hop album of the last five years hardly made a splash. Rolling Stone gave it a modest three and a half out of five stars. It appeared fleetingly, arriving unheralded and departing quietly like a specter in the mainstream consciousness.

Last Sunday, Feb. 10 was Dilla Day in Detroit.  Hip-hop fans poured into the Filmore and they didn't leave until after 2 AM.  Brooklyn hip-hop icon Talib Kweli and Detroit's own Royce da 5'9" were the...