Some people speak to the power exchange/reversal, but my own preference is a bit oblique.
Tough girls make for interesting reactions because they're not used to having impulses overwhelm their control. So you get a tough chick who's really ticklish and can't control it, their reactions will be more erratic than say, fragile girls who are used to that sort of thing and usually react in demure ways.
Tough chicks also tend to not be so damned sensitive about things unless they feel violated. A tough girl might not like being tickled because of the potential loss of control, but if they find the feeling to their liking, they ain't usually shy about getting it again, while traditional girls tend to do the whole play-acting thing hoping you will decipher their non-verbal cues and get the drift, which I fuckin' HATE.
And finally, tough girls tend to have a good dose of body types that aren't cookie-cutter, assembly-line Cosmo-cover attractive. They have meat in areas most girls don't, and that sinewy quality can be extremely hot; no offense to classically attractive skinny girls, but muscles can move as finely as slink with the right "motivation". They also tend to have deeper voices, and the laughing in that voice can wreak glorious havoc on your own self-control.
It also feels far less rape-y knowing that the chick you're tickling could rip your balls off and turn them into earrings if she wanted to. At least then there's the security of knowing that you can't cross the line without being made aware.