[Petey1016]

well in the fluff its is known that Slaanesh knows all of your darkest secrets and desires i think that is why he/she is called the Keeper of Secrets. Though you did raise a good point about The Changer Of Ways, i think they thought mostly that Tzeentch changes his worshippers more than other gods do (mentally as well as physically). Nurgle turns you into walking Puke which is all Physical and Khorne turns you into a Mental killing machine which is more mental. Slaanesh is more mental change into a deranged Paedophile/pervert only seeking pleasure. Whereas Tzeentch does both to the same degree.

Thats my Two cents.