One that made me feel like I had wasted time watching the show was How I Met Your Mother. It was a fun show and really picked up in quality in seasons three - five. They could have ended the show in season 7, really, if they had planned for it. Season 8 was kind of a bore, and season 9 was bad, as all of the season took place over a weekend. And when we got the finale of the show... I was so tired of holding on to what might come that it really hit me negatively how they ended it.
That being said, Game of Thrones ended so poorly that I was baffled as to how haphazard and dull the writing and storytelling was that I, just like a lot of others, held on to hope that the last few episodes might bring things all around. (Morgan Freeman as Narrator - It didn't.) While cracks were showing since late season 6, the finale of that show was horrible. The payoffs didn't come, and everything just felt so rushed, watered down, and a tremendous feeling that content was missing from the season, if they were to help make sense of the finale.

