Yeah I didnt like RA3 much either.

I can't recall anything that would really fit the thread, but my biggest disappointment might have been buying Brothers - A Tale of Two Sons thinking I could play it with keyboard controls or DInput PS2 controller. It looked good and I really wanted to play it, but repeating the beginning countless times, fiddling around with the controls, then trying to use some third-party program to emulate XInput, and failing again and again to make it work in an acceptable way almost killed my initial enthusiam, I was sorely disappointed and even resented the game for it a bit. I had been aware that the game was meant to be played with a gamepad, but at the time I only had those PS2 controllers and not being an expert on gamepads, I naively thought those combined with an adapter for use on PC would be enough. And when it turned out it wasn't, I read that keyboard controls are supposed to work, too - some still say they do, but for me the first challenge with the elevator winch right at the beginning made them seem frustrating to the point of game-breaking. This disappointment was so big that at some point when I was in a bad mood for other reasons, I grudgingly bought an Xbox controller to finally be able to play the game. That was pretty extreme, but I never regretted *this* purchase at all. I still think you shouldn't have to buy a gamepad to play a game on PC, but I also learnt that it can be fun to use one, and I've grown quite fond of it now and use it for lots of other games.

A game that was an actual letdown for me, although not at such a dramatic level, was Richard and Alice. At the time of its release it was highly praised for having a mature story or something, and I bought it in the hopes that it would be special somehow, and maybe even close to Wadjet Eye games or To the Moon, but I ended up hating it. People criticized the graphics, but as a fan of AGS freeware, I thought graphics were secondary and I was ready to believe that it could be great regardless, but gameplay and story/writing are actually much worse than the graphics, IMO. The dialogues are very longwinded, you just click and click and click through them as if it was a visual novel, not what I thought the clicking in point-and-click adventure was about. Puzzles are few and simple, but the walking speed is so slow that it makes everything feel like a chore. The story is incredibly bleak and kind of pointless rather than mature, it didn't really offer any food for thought to me. But meh, whatever. Fortunately, most people might not even have heard about the game, and I think even if I overpaid, I did not pay much for it.

you should play it classical and not with the new revamped graphics and all

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My biggest let down game must have been

Final Fantasy XIII

After 70 hours realizing you'd still had at least 40% of the world to go was a bit to much, after the first try i never managed to get myself worked up enough to give it another try.
Final Fantasy coming to the PC for the first time was a great happening at first, oke there where some graphical problems, for AMD users i believed, but yea there they where like a shining knight from Japan bringing us an exclusive not meant for sharing in the first place.
Oke, the first time entering the world was pretty amazing, in those days i still had a surround set connected to the pc so with the audio and the amazing meticulously made movies, Lightning was a pretty cool character, a S.O.L.D.I.E.R. .... lol and the first time having a go with the turn based tactics was quite amazing, fights felt pretty lifelike without, at start getting anywhere near a rushed feeling. Of course the like what 30 hours tutorial had some more surprises in stall after which the game actually went sour quite fast. The story nor the movies where anywhere near content that could inspire some ridiculous taste for the next round of this dinner party not to mention the prospect of juggling with supersized monsters for at least 10 times an hour.

what a disaster that one truly was!

Overhyped in terms of expectations vs the end product, I'd say my biggest letdown was Oblivion.
I've come to acknowledge over the years that the game has its strengths, but off the heels of Morrowind it was inferior in every way that made Morrowind great: I was incredibly confused how Bethesda could have made it as a sequel to that game.

Ultima IX: I got the collectors edition even. It was such an utter disappointment that I never put another game on a wish list ever again.

Two of my favourite games :(. Follow it up with "and I totally didn't get what all the fuss was about Grim Fandango" and shove a knife through my heart!

Myst
i never play a game more stupid then this what a waste of time.
it's good it's death buried and forgotten in the myth of time

I was quite hyped for Aliens: Colonial Marines. I've played the release version - to say I was disappointed would be an understatement.

Darksiders III:
I was super hyped when it was first announced and i saw the first gameplay of it, but it ended up playing slow and boring with unsatisfying combat. That being said, it redeemed itself later with an added mode that makes the combat more classical. After that update, it played a lot like the Castlevania: Lords of Shadow games which i love. In the end, i'm still slightly disappointed with the absence of executions though.

Monkey King: Hero Is Back:
In theory i should love that game. My problem is simply that the combat is boring. There is not enough you can do. I expected better than that.

Agents of Mayhem:
I love the Saints Row games and i like the idea behind Agents of Mayhem. It just ended up being terribly dull and forgettable.

My biggest let down are:

Duke Nukem Forever: one of the worst FPS I ever played. Never finished. I expected a modern version of Duke Nukem 3d: what a terrible mistake...
L.A. Noire: extremely boring, it could be simply more in every aspect except graphic.
Rage: I expected much more from ID software. It's boring nonetheless and there were much better games placed in a post-apocalyptic world.
Outlast: too much frenetic and some scenes with gratuitous violence (like the two fingers part). I prefer much more Amnesia, less violence and much more fear.

"A View to a Kill" on the C=64.
What an abomination.

LotR2 was the reason I came to GOG. But the game just didn't work like it should.

Thief 3 just wasn't what the others were.

I wish Thief 4 didn't have the sex because a) it was pointless and unavoidable and b) the rest of the game isn't that mature. I'd love to share it with my kids (my eldest loves Thief 1), but no can do for a long while.

Probably Deus Ex: HR. I got bored and never finished it.