While Act I and II had their flaws, Act III and IV were just absolutely atrocities. The two main reasons being Azmodan and Diablo.
"I don't neeeeed my siegeweapons to destroooooy youuuuuu"
"Oh you destroyed my Heeeellrift, but it doesn't matter, because I have another Heeeeeeeeeeellriiiiift"
Saturday morning cartoonvillains is not good writing... Really, just stop them from talking and taunting you constantly. Also, Diablo with boobs was still fucking stupid.