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

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "kicker":
   astonishment, bellyacher, blockbuster, blow, bomb, bombshell,
   boundary condition, catch, clause, complainant, complainer,
   condition, crab, crank, croaker, donnee, earthshaker,
   escalator clause, escape clause, escape hatch, eye-opener,
   faultfinder, fine print, frondeur, given, griper, grouch, grounds,
   grouser, growler, grumbler, joker, kvetch, limiting condition,
   malcontent, murmurer, mutterer, obligation, parameter, peripeteia,
   prerequisite, provision, provisions, proviso, querulous person,
   reactionary, reactionist, rebel, requisite, revelation,
   saving clause, shocker, sine qua non, small print, sorehead,
   sourpuss, specification, staggerer, startler, stipulation, string,
   surprisal, surprise, surprise ending, surprise package,
   surprise party, switch, terms, thunderbolt, thunderclap, ultimatum,
   whereas, whiner