Hellgate: London is best described as a spiritual successor to Diablo, one that ditches the isometric perspective for a 3D viewpoint and a more futuristic setting.
This is of note only because Flagship's founding members, including CEO Bill Roper, were largely responsible for Blizzard's best-selling Diablo series.
Hellgate: London is the first release from developer Flagship Studios, a company formed by key executives from Blizzard North.
