I wholeheartedly agree with Milagros on this one. To me, a bad 'ler is one who is utterly imperceptive. One who has no ability to "read" their 'lee. I guess that's why I find men who are overly perceptive a HUGE turn-on (whether tickling is involved or not). Guys like Hugh Laurie's character "House", a man who notices even the most subtle nuances of those people around them - SEXY AS HELL.
That is the kind of guy who is going to notice every twitch, gasp, or giggle as they begin to discover what gets the best ticklish reaction out of you. With that kind of guy, even when you're not laughing outright, he KNOWS the effect he's having on you, and it is only a matter of time before he gets the reaction he's looking for. Any attempt at deceit or concealment on the part of the 'lee is pointless because he is looking beyond your words, watching for the time when your body betrays even your most well-guarded secrets.... despite your best efforts to the contrary.
Since I am a 'lee who makes a game out of concealing the effects of my 'ler, and makes every effort to hold back my laughter for as long as humanly possible, a perceptive 'ler is not just wanted, it's needed - otherwise, it may be assumed that I'm just not ticklish at all.
And if you couple all of the above with a great sense of humor, and a playful spirit, even the most stubborn 'lee doesn't stand a chance.