The plot pacing was much better (since it didn't just teleport characters around Westeros wherever and whenever they're needed to advance the plot, faster than characters could actually travel), the plot itself was way better, mostly 'cause it took way more interesting and brave decisions WITH its plot (such as killing off pivotal characters which, y'know, fucking made the show this popular and intriguing in the first place), and the biggest part for me at this point ('cause you kinda can't stop the ending of the show from bringing characters together and losing the great world-building for the most part and making it a big humans vs the Others) is that the dialogue just isn't nearly as good and it doesn't have almost any clever quotes that even approach the great ones that the show adopted from the books.
Also, a lot of characters who are meant to be clever and smart people (Littlefinger, Tyrion, Varys) were butchered and written into corners, because an author can make a character only as smart as they themselves are, and D&D are clearly much less intelligent and imaginative compared to ol' Georgie.
Also, the plot NOW (in season 8) is alright for what it needs to be (i.e. defeating the dead and Cersei and wrapping shit up), but a part of the show being terrible after the books ended is, umm...the whole Dorne storyline, which was unbelievably retarded, whether you know how much better it is in the books or not. It was just HORRIBLE altogether.