Shock value comes first to my mind.
Then if changing stuff is awesome, the show does not deserve to be called "adaptation of the novels" (as it is the case), but "inspired by the novels". Which is not the same thing.
If I'm watching an TV show / movie adaptation of a book, I expect it to be true to it, and that's how I will judge the adaptation (i.e. if I like it enough to buy dvds of it for example).
I am not that opposed to deviations from the books, as much as they make some sense and are correctly handled.
Some deviations were handled the right way, and led to amazing scenes : the first one that comes to my mind is all of the Tywin-Arya scenes in s2 that were absolutely perfect. I was initially angry that Sansa went to Winterfell instead of "Arya" (Jeyne), but that lead to a change in Brienne's story (which I'm pretty sure that it was boooooring in aFFC) and made it interesting (the fact that Jeyne was scrapped in the first season also helped), thus I am beginning to think it was relatively well handled (also, Ramsay's the best, more Ramsay is good).
Some deviations were handled poorly, and led to pure and utter shit : Missandei & Grey Worm which is a waste of time, Jaime freeing Tyrion without mentioning Tysha (which was a turning point in Tyrion's life), Drogo raping Dany in the beginning of s1, Barristan being killed, Valyria being shown as a jungle whereas it is supposed to be a scarred and burned wasteland ...
I cannot enjoy the show if they keep butchering the books and the lore like they are doing right now.