At the time this came out, 1st person shooters were very common. Quake, Quake II, Unreaql, all promising great action, great engines, etc....but then came Thief, sneaking into the market as stealthily as Garrett himself.  For those that had the fortune of trying out this unmarketed little game got something they have never seen before...a 1st person action game that forced them to think.