From 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]