It's not as much the unrelenting grimdarkness that's getting to me as much as all the deaths starting to feel gratuitous and empty of any meaning. From seasons 1 to 3, it was fine, considering that the characters who were on the line were characters we had spent time with, and who the viewers had grown attached to. Their deaths had weight, the editing made it so that it wasn't taken completely casually like the deaths of Roose or Doran this season. I mean, just compare the death scenes of Ser Rodrick, Maseter Luwin, or Jory to the ones of the characters I just mentioned, the difference in tone is just baffling.
They've run out of disposable characters people care about and who are in realistic and emotionally involving danger (mostly because they cut off so many characters from the original story in the first place), but they can't do away with the "gimmick" of characters still dying regularly, and so all minor characters are sent to the butcher, even if that might not be their fate in the books.
As far as the ugly matter of rape is concerned, I understand why the writers did what they did to Sansa. Her chapters in AFFC are not TV material (and would have required them to introduce a buttload of characters that should have already been known to the audience by now), it kept one of the better storylines in the books, and it was pretty much the logical conclusion of having her at Winterfell. What I cannot excuse though (and is often overlooked), was the rape of Cersei by Jaime by the dead body of their son back in Season 4, it was completely gratuitous, out of character for Jaime (who is one of the characters who despises rape the most in the books), and only meant to shock. It was absolutely disgusting.
Concerning the current season, I thought the first two episodes were complete garbage, that the second was dull as sin, but I liked the last one, it was surprisingly good, even though the ending was mindbogglingly stupid.
EDIT: I'd also like to add that most characters are now parodies of themselves. Tyrion can only quip, Brienne lost all of the vulnerability she had in the books (though she never had much of it in the show), Dany is always angry and defiant, and so on, and so on.