Sword of the Stars II was the one that let me down. also Star Trek Online was another bad one.
That’s not germane to the thread, the OP specified games; not high-concept betas or natural disasters. 
GTA4, I seriously regret paying for a game where I get calls from whiny bitch “friends” who want to play shittty mini games or they wont be my friend anymore and a cousin who calls and says “COUSIN, its your COUSIN”
every
fucking
time
I never finished it, tried to kill my cousin in a myriad of ways and when I couldnt I just stopped playing.
Mass Effect. Just can’t get into it that much.
Dead Space 2. The first one was novel, creepy as hell, and rock hard. Even playing it now I get goosebumps and nerves when I turn a corner. The second game was EA’d into a doomathon boo-fest. I haven’t picked up 3.
Zelda: Wind Waker. I actually found this hugely underwhelming back in the day. It was gorgeous and the characters were likeable, but the ‘huge’ world you sail upon was a trick: it was specks of tiny islands with not a whole lot in between, and the Triforce fishing was dull, dull, dull. And the game had fewer dungeons too!
Oh, I’m also underwhelmed with modern racing games. I know Burnout: Paradise started the craze (I think) with having an open world city to drive around and choose races based on where you are, but I actually find that kind of system horribly awkward and tiresome. I much prefer the game just giving me a race, old-school style.
So when I bought Need for Speed: Most Wanted 2, my first racing game in absolutely years, I was sorely disappointed that this open-world mechanic seems to have become a staple.
Prince of Persia: The Sands Of Time. This one might be a little unfair, because I started out with The Two Thrones, but I kept hearing that Two Thrones was a step down, and that the writing and characters were so much better in Sands of Time. Then I booted up sands of time and found the characters to be meh and every part about the gameplay to be significantly less interesting. Maybe the people I heard that from were wrong, I dunno.
One Finger Death Punch. It looked sooo awesome on Youtube. After an hour I got really bored with it.
Spore of course. Titanfall had the potential to be great but just fizzled out.
Anyone evey play Midwinter? It was supposed be a huge open world in which you recruit followers and send them on missions, conduct guerrilla warfare etc.
In the end its easier to just load up on explosives and do it all yourself. 
DS3 is like 2 - only more so. Really, really boring.
Dark Souls 2.
I was not expecting such a U-turn from the previous two games. Even overlooking all the flaws of gameplay mechanics and level design, they managed to make it a graphical downgrade and a worse PC port. I would’ve thought the latter would be difficult to accomplish, but they managed it.
Ooh, another: Twisted Metal 3.
Yet another example of a sequel (this time to the all-time classic Twisted Metal 2) being more advanced, better-looking…and yet somehow less playable and more boring. 
I love the Mass Effect series more than any other game(s) in the world, and I constantly tell people to play it. But I always preface it with "Slog through Mass Effect 1 just to get the story and decisions so that you can play 2 which is a video gaming masterpiece. Then play 3 because 3 is fun and I liked it.
Besides if you buy just 2 you might get the fun comic thingy that lets you make some decisions.
I’m gonna put in Twisted Metal. The newest one that came out (not the original). Black was fun enough, but this one changed what it means to be in the TM tournament and I was not impressed/did not enjoy it
It’s funny how we have a couple people feeling the Mass Effect letdown, whereas I’m over here feeling terribly let down by Dragon Age: Origins. I like sci-fi. I like fantasy. So I should like both game series. But when my companions in Dragon Age kept blabbering and blabbering and wouldn’t shut the fuck up for five minutes, that was the end of it. Every time I get a real-time character talking in a game I know I have to stop completely if I want to listen to them, because heaven help me if I continue and the dialogue gets cut off by a battle or a different line of dialogue. I felt like I was standing around in towns forever, waiting for these people to stop talking so I could move again. Held hostage by too much character development. And the fucking dog stuff. Someone talked way too much about dogs. And then someone else started monologuing about cats.
That’s what I remember, but the last time I played was 2009 when it came out so it could be warped.
In contast I’ve spent the last month replaying the Mass Effect series from beginning to end, all at once, and I’m loving every minute of it. It only gets better as the series goes on. Whatever rocks people’s boats. I wouldn’t have thought Mass Effect and Dragon Age would have a dividing line for most people.
Black & White and MGS4 were also big letdowns, just like everyone else. Tales of Vesperia. I slogged through it but it’s definitely my least favorite Tales game. Surprisingly not grabbed by Dark Souls. Loved Demons’ Souls. Don’t know if “let down” is the word for it, but I wasn’t feeling the undead thing and estus flask at all.
As mentioned above, for me it’s SimCity.
EA simply CANNOT give the people what they want: A natural evolution of SimCity 4.
We want massive, customizable regions that interact in every aspect.
We want detailed terraforming.
We want to custom-design streets and intersections.
We want huge, processor-torturing cities.
And so much more.
And, we want it all to work online in massive multiplayer.
That is the natural progression of SimCity and they absolutely refuse to give it the gaming community.
Instead, they insult our intelligence and repeatedly foist crap upon us like SimCity Societies and the new SimCity like we are amnesia-laden imbeciles.
They are in denial. It’s frustrating and perplexing.
I have another one: Destiny.
Now to be fair, I only played the “beta,” which was actually just a demo.
Very pretty game. Good voice acting. Decent controls. But at the end of the day it’s just Borderlands without soul. Real letdown for a game that I’ve been casually following for quite some time.
I sort of agree on SimCity, but really, you can dump the multiplayer. I want to build a gigantic city, with cool stuff in it. I wanna turn a crossroads into New York. Why they don’t do that I do not understand. I don’t care about YOUR city.
One of the weirdest decisions they ever made was actually in SC3, when you could place any one of dozens of awesome things like the CN Tower or the Sydney Opera House… for free. It just sat there and did nothing, and cost nothing. You could have a crappy little 10,000 person city with the world’s greatest skyline. I could never understand why there wasn’t a COST associated with that, and a corresponding benefit; make the Sydney Opera House cost you $200,000 (a massive amount in SC3) plus a huge running cost, but it increase commercial and residential demand by 15% and has a huge property value boost for X squares around it, or something; the kind of thing you need an already big city to afford but which would help you get bigger.
SC5 (they don’t call it that but whatever) just baffles me, and EA’s decision to lie repeatedly about the game immediately following its release does not fill me with confidence for the future.
I agree. If ever there was game that didn’t need multiplayer, it’s Sim City. Hell, the fact that Sim City 4 kind of forced me to let my own cites interact with one another was annoying to me.