That's not a bad thing.
Season 1 was pretty close to the books, but they took some big liberties in seasons 2-4 for time and made some decisions I didn't always agree with. Also, pacing. Martin's pacing isn't suited to television, and while the show worked around it reasonably well with the material they had, season 6 really shone because they could finally pace out the material in a manner that makes sense natively on the screen.
Book-to-series adaptations are always difficult, but ASOIAF struggles with its pacing and hugely irrelevant forays even in the novel format. Diverging the books and the show will do nothing but make both better, and I think it already has (Battle of the Bastards being truly exemplary of this).
What I mean by Martin's writing is good dialogue, characterization, and plot. Not pacing or whether his writing also involves irrelevant forays; obviously, in an adaptation pacing is changed and unnecessary material gets cut, that goes without saying.
My problem with the show's writing is primarily how there's plot holes everywhere, characterization is inconsistent, valuable time is wasted on tits, stupid jokes and fanservice, how the dialogue gets bafflingly anachronistic at times, there's just plain continuity errors and the focus is so heavily on constantly trying to shock the audience and having big moments at the expense of plot logic.
I'll never complain that they shouldn't have cut this or that secondary character or plotline,
except when they're replaced with something that's obviously worse than what the source material allowed for. For example, want to cut Dorne? Sure, fine, there's only a limited amount of screentime per season, you need new locations, quite a few new characters. Perfectly understandable if there just isn't enough time for it. But it's completely inexcusable to have it and then do with it what they actually did. With the same amount of time and resources, they could have just adapted the queenmaker plot and made it ten times better than what we got.
As for the BotB, for me it was basically pointless fanservice spectacle, as the plotlines leading up to it were nonsense. Sure, it was visually wonderful and there were some really pretty shots, but what's the point if the story underneath doesn't make sense? Technically impressive visual spectacle is a dime a dozen these days, and if I had to choose, I'd much rather have a better story with the battle happening off-screen than a bad story with an impressive on-screen battle. Blackwater and Watchers on the Wall were much better episodes, not because the battles were more spectacular (well, the latter was), but because they made much more sense so I was able to actually be invested.
