Well, I think slash is subversive because is nit (ot, wasn't, at least) what authors were aiming. So, we're subverting the text, making it ours by changing what it means, finding sub-text and extracting conclusions from there.
But is the sexual aspect which, IMHO, makes it really bothering for TPTB? society? general others?
Homosexuality has been a taboo in TV for almost its entire life, and take famous characters and show them as in a same-sex relationship was, in a moment, very, very bad. We, slash girls, must be ashamed from ourselves. Women enjoying male-male sex were considerate almost pervert and most of writers and readers hidden it from family and RL friends.
I know, I did it.
Today, most of this issues are not real. Slash has become almost a standard in fanfiction, and with writers actually telling us they are writing sub-text, that aspect also is less subversive. Of course, you could see other pairing, or see it in a show when it was not intended, but is not the same.
Sure, there are others subversives aspect in slash: men more in touch with their feelings, relationships where both partners are equals (we can say men and women are equals, but, seriously, when we come home from work, who do the dishes, and laundry, and almost anything? Who ask for days at job when kids are sick?)
But for me,the real "wicked" aspect in slash is... women: we are women, women who enjoy sex, who writes sex, who thinks two man together are hot and, worst, we think we have the right to enjoy sex, write sex and be playful about it.
So, there is not a simple answer. There is not only one answer. As anything worthy in life, there are more views than viewers =D
BTW, english is SO not my language, please, forgive my horrible grammar *blush*