I've tried to make the subject as broad as possible to include everything but the core of my thought is what movie, artist, television show, one hit wonder, actor/tress, series, whatever made you feel disappointed after. It could be from the initial hype or from where it started to where it eventually ended up or however you interpret the question really.

A few for myself: The second Boondock Saints movie. It was not that I disliked it but more that I enjoyed the first one and the gap between the two was quite significant that the build up for it finally being made was a letdown.

Super Troopers again not bad for the series but large gap and kickstarter campaign ultimately for an alright movie

Is It Cake? Enjoyed the first season but 5 minutes into the second season I was bored and tired of the idea already. I turned it off. The jokes felt terrible, the hosting felt bad, and the concept was very boring suddenly for a show I really enjoyed the first time I watched. 

Game of Thrones and Line of Duty were both tremendous disappointments in how they concluded. I really hope Stranger Things nail the finale but I am very concerned given how much they have to wrap up that they are going to miss the landing, and we end up with 2 crap movies to follow like they've done with other shows. Fingers crossed.

Coincidentally the only episode I ever saw of Game Of Thrones was the final episode. I knew it was very popular but having watched the finale it felt very underwhelming. To the point I was happy to have never seen the show prior. 

Another television program that I enjoyed, but if I am honest about how much I enjoyed it at the start versus the end, was Mr. Robot. I have watched it completely a handful of times, but there are some episodes where I question if I am watching the same show. So much so that if the middle to late episodes had been earlier I may not have watched the show to begin with. 

Stargate Universe was the biggest letdown. Two seasons. Ending was no ending. No one knowing what happened. Then all bad-ass shows that were on the Sci-Fi Channel goes off the Sci-Fi Channel to never be aired again.

The Walking Dead (11 seasons)

Fear the Walking Dead (8 seasons)

Teh Walking Dead: Daryl Dixon (3 seasons)

The Walking Dead: Dead City (2 seasons)

The Walking Dead: the Ones Who Live (1 season)

I am not sure if the number of seasons reflects the respective quality of the subsequent spin off series 

it seems every new series/show is setup to milk whatever IP they can create into oblivion for money and scripted that you have to watch them all for 5+ years to even get any hint of whats coming next. we need good mini series again

Need companies to become absorbed into larger companies. There are so many options of where to watch anything anymore that no longer can you focus on any one service. Having so many choices is fine, however when it comes to creating new content it is quite difficult to remember which service has which program. Also the talent pool for actors and writers is quite shallow, so you receive half-assed shows with half-assed talent. At least previously, because so few channels actually had content worth watching, you knew about the show you wished to watch.

Let me phrase it like, since at no way/shape or form am I trashing the show, this is purely subjective - I never-ever liked The Breaking Bad that much at all, I watched a bit and I stopped, for no reason or whatever but I never came back to it.

The final season of Killing Eve. Dislikeable new characters, a slow moving plot. They should have cut it a season early and had one of them throw the other off Tower Bridge. Would have worked much better.

SteveTheJobless - Very much agree with your here. Gave the show a try when it first came on, maybe a little after it had started, and had a major problem. The wife of Walt drained the life out of me every single episode. So much that I turned it off as I could tolerate her no more. Not sure if I would have enjoyed the show without her existing but certainly was incapable of enjoying with her. 

BuddyBigPaw - Never saw the show although I have seen previews and such for it. It is unfortunate when a show goes past where it should have ended. A point which does seem to occur more and more but I guess they need to squeeze every last bit of profit out of a show.

Oh gosh, so many book-to-screen adaptations have been incredibly disappointing, so it's hard to know where to start lol. But His Dark Materials was such wonderful source material and they never quite got it right, for one. Game of Thrones was definitely another, the ending at least. And Suits could have been good but ended up being bleh. 

Two of my favorite all time shows went from amazing to damn near unwatchable after a certain number of seasons. The Walking Dead went south much quicker than Game of Thrones did, though. I think TWD became unwatchable somewhere during the whole Negan phase, as they dragged that shit out WAAAAY too long.  As for GoT, the last few seasons were slower, but still watchable. The final episode was a huge letdown, though.

It has got to be series 2 of The Night Manager. I was SO excited to see it return after a decade, and the first series was absolutely captivating. Sadly, for me, series 2 was almost a complete copy in a different country. I get that it's the villain's M.O., but it just didn't have that edge.