Right off the bat, I want to point out that Game Of Thrones had such a wide audience that there are myriad different opinions on how good its ending was and what it should have done. My answer is but one of many possible interpretations. However, given that you're asking for a spoiler-free answer anyway, it seems that I can navigate away from the murkier subjective waters of which parts of the story could've been resolved better, instead sticking to the general consensus in most online discussions on the topic, which I believe to be sufficiently objective for the purpose of answering the question.
TL;DR The resolution of the story wasn't bad, but it was told in a hasty manner. There was a significant drop in writing quality and attention to detail, when compared to earlier seasons.
Imagine if I want to write the final book in my book series. I'm going to wrap up the stories. After writing, it turns out that it's a 3,000 page novel. But my publisher limited me to 300 pages, no more.
Rather than rewrite my story or condense its narrative, I decide to simply rip 2,700 pages out of my book and release the remaining 300 pages. Readers are missing a lot of subcontext, and the story seemingly jumps from one plot point to another without giving you any breather inbetween or building up any narrative.
This is, at its very core, what went wrong in the last season. Too much content to wrap up, not enough screen time to walk you through it, thus rushing you past the needed exposition and foreshadowing. The lack of exposition and foreshadowing in turn made the conclusion feel like a lampshaded and forced twist, rather than the natural ending to the arc.
I won't discuss any specific arcs because of spoilers for obvious reasons.
In the end, I think the completions of most arcs would have been much better received if there had been more storybuilding to lead up to its conclusion. I don't think the arc resolutions were bad, but it required the viewer to make major leaps to understand how the characters would go from [end of season 7] to [end of season 8].
The distance between these two positions was big (for some arcs - not for others), and there simply wasn't enough screen time to actually walk you from point A to point B. Based on the opinions I read online, the negative focus was almost exclusively on arcs that were very different from what we expected them to be (at the end of season 7).
You, or others, might disagree on a per-character basis whose arc did or didn't make sense, or felt rushed, or did not end in a satisfying manner. But that's very subjective and not meaningfully answerable.