Good question! There's so many facets to tickling, it's hard to narrow down to an easy answer. For me, tickling is abit like my body betraying me, in that, I can take a decent amount of pain, but am rendered mush by simple stroking of the wrong(right?) areas. Take the feet: I will totally spazz-out if they are even lightly touched.
There's alotta styles to tickling, too. Most of the ticklers I've ever played with tended to go for the kill, with lotsa fingertip gouging into areas like sides and ribs. It's a guaranteed hot-spot, and it'll get me thrashing, but for once, I'd like one to take it in increments. Slow, barely there touches(more like foreplay) for abit, to get the motor humming, and then build up to the crazy gouging and poking attacks.
Hope this helps some with the question.