The Collaborative International Dictionary of English v.0.48:

Kicker \Kick"er\, n.
   1. One who, or that which, kicks.
      [1913 Webster]

   2. A fact, condition, or circumstance, sometimes concealed or
      not obvious, which reduces or eliminates the benefit of an
      apparently advantageous situation; a joker[5]; as, under
      the Soviet system, bread was good and cheap, but the
      kicker was that you waited in line for hours to get any,
      if it was available. [informal]
      [PJC]

   3. Hence: An unforeseen added expense or additional cost; as,
      the printer was cheap, but the special paper it needed was
      an expensive kicker.
      [PJC]

WordNet (r) 3.0 (2006):

kicker
    n 1: a player who kicks the football