Debating show endings is a fun enterprise if not a fool's errand. Other than some particularly egregious examples, it's a matter of preference. Some like complete and total closure to every aspect of the show. Some like vagaries. Others like an open-book ending that allows them to fill in the blanks with their own interpretations.
Normally, I'm one for closure. I don't want to have to use my own imagination as to how things end; I daydream when I want that sort of control. You started the story. Finish it.
GoT gave us as much closure as we could have reasonably expected, so I can't complain about that aspect.
However, regardless of what sort of ending one prefers, I believe it should go without saying that it should be well done and flow naturally, with solid provenience to the story. That's where GoT fell short for me.