(n.) One who, or that which, kicks.

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