To clarify: disappointing does not necessarily mean bad. Sometimes disappointing things can just be average, and on occasion, they can be really f**king good - it's all relative to expectations.
At this point in its life, Thrones is the single biggest show on TV, with any claimants to its, er, throne either falling by the wayside (The Walking Dead) or still too young to tell for sure (Stranger Things). More than that, though, is that Thrones is typically one of, if not the, best shows on television. And with Season 7 being slightly delayed and also its penultimate, there was a lot to be expected - and it didn't fully deliver.
Yes, there were great moments. The spectacle was as grand as ever, with the dragons and White Walkers unleashed to great effect, and some lovely character moments too (the first meeting of the Eastwatch Seven, Arya and Brienne, Gendry's return and so on). And that makes it all the more frustrating that this season was only seven episodes, and as a result almost everything felt rushed.
Storylines that would've taken whole seasons zipped by in a couple of episodes, certain motivations and conflicts didn't quite fit with the characters experiencing them, and the show took Wun-Wun sized leaps of logic in order to cram everything in and establish the endgame. It was still very good, but rushed storytelling and big logical gaps shouldn't be a feature of Thrones, and it's ultimately a disappointing year for the show because of that.