Well you've opened a can of worms with this thread...

The Mass Effect Sequels - Not just because they're bad sequels and bad RPGs but because they're good third person shooters, which means people don't understand why I hate them so much. There are hundreds of shooters, Mass Effect however was my AAA sci-fi RPG, name me one other AAA sci-fi RPG of that gen... there aren't any. The closest would be Deus Ex and that's cyberpunk, completely different feel to a space opera. How many third person shooters are there though, fucking dozens. The changes may not have been as massive as I perceive them but it positively screams "sell out", and I will never forgive BioWare for that.

Final Fantasy XIII - The single most disgusting game I've ever played and again, the fact that it's competent at least makes it worse. Yes sure there are worse games, but this took the name, good will, and standing of a franchise I love and sunk it to the very depths of mediocre pap and fan-service bullshit. The storyline is senseless, the characters moronic, the setting inconsistent, the pacing glacial, the gameplay non-existent, and the focus perverted. This games art direction budget alone is several thousand dollars too damn expensive. If they cared as much about making decent games as they do making Lighting look hot on billboards to sells shit to idiot Japanese fanboys, maybe the series would still be worth a damn. FFXIII is an insult to the franchise, but then everything Square Enix does is a bloody insult now (Eidos doesn't count).

Fable 3 - The Fable franchise is somewhat of a guilty pleasure of mine. They're not especially good games, but they're not bad either if I'm honest and I did genuinely enjoy Fable 2. While it has massive flaws and is painfully shallow at parts, it is actually quite fun and I can respect that they wanted to focus more on certain aspects of the game. Then Fable 3 happened. How did they fuck up so much, the gameplay felt weak, non-gunpowder weapons are next to useless, spells are too damn powerful now, and the storyline jumps around like crazy. Pacing is a concept this game just doesn't get. Fable 2 is a guilty pleasure, an honestly rather shallow game that I have far more fun with than it deserves. Fable 3 took all the little redeeming things about the previous game and ruined them leaving a sub par fantasy action-RPG.

Fallout: New Vegas - This was such a disappointment. Fallout 3 is awesome and the world has so much potential. Then the DLC came along and fixed every story issue I had and expanded the game even more. Brilliant. New Vegas lands and it's the most glitchy buggy unfinished mess Obsidian had done to date... and considering they made Alpha Protocol, that's saying something. They actually broke shit that worked in Fallout 3! How do you do that? If the code works, leave it the fuck alone. Obsidian touching something is basically an admission that it's about to fall apart. They are quite possibly the most incompetent developer in existence... and the bastards keep getting handed franchises I like. Even if we ignore the glitches though, there's more. The game punishes exploration. The desert wasteland feels empty, until you're stupid enough to shoot something then 300 enemies pop out from behind the nearest rock to descend on your arse. The storyline is railroading and boring, (I don't care about the damn chip or the guy who shot me). The world isn't realised at all... hell go to Gomorrah, it's built up throughout the whole game as this den of depravity and awesome fun, and when you get there it's boring, bland, and fucking empty. There's no-one in there, there's no atmosphere, just a few slots and a blackjack table that causes the game to crash. This is an insult to the franchise.

Civilization V - This is one that I genuinely tried to like but I couldn't. They removed half the civs, the government system is pathetic; it's nothing but World Of Warcraft style talents and has no feeling of tactics or choice. Abilities from Communism and Democracy should not stack, and you don't progress through one to the other, that's just stupid. There's less tech, there's less buildings, there's less customisation... there's just less. The only thing they improved was the combat system and while that's great, if I want a more combat orientated game I'll play the Total War games. Civ was always about expressing yourself as a world leader, this game didn't let me do that. There was a forced line of progression and very little flavour. For someone who's never played a Civ game before it's likely incredible, for people who know better it's a massive step backwards.

Killzone Shadowfall - Its not a bad game, I just expected it to be on Killzone 2 levels.
Farcry 2
Fallout New Vegas
Crysis 2
Assassin's Creed 3
Medal of Honour: Doorfighter
Shift 2 Unleashed
Battlefield 4 - Biggest disappointment ever!
Grand Theft Auto 4 - Second biggest disappointment ever!

& every Call of Duty since Modern Warfare 2

I never let myself get disappointed with a game, particulary because I try going with a clean state of mind and no hype when playing.

That being said, my biggest disappointments are:

Bioshock: A FPS game that is, by no means, the quality of System Shock 2. Interesting story and setting (good atmosphere too) doesn't make this dull shooter much better. I've never understood the appeal of a character that five minutes into the game is inyecting himself with some syringes (And YES I KNOW WHY HE DOES THAT LATER IN THE GAME), that suddenly becomes too OP and indestructible. The prepotence of a certain character didn't help in my opinion, though everyone else seems to love that char. Not a bad game, but certainly not the Opus Magnum I've heard. I actually enjoyed Bioshock 2 inmensely more because everyone hated on that game, while I think the quality is superior.

El Shaddai: Ascension of the Metatron. A shame of a game. I actually enjoy the artstyle, and the Biblical-based story was an amazing touch. Everything else fals appart. I'm no fan of hack n' slash games, and certainly this game is barebones with that system. Hiting the same enemy five minutes for him to die (Not to mention having to purify your weapon every two minutes) isn't my idea of fun. Still haven't beaten this game, and it is probably one of the few games I own I've never ever beaten.

God of War 3: One of Sony's most iconic franchises, and the first GoW I play. Didn't like it one bit. Impressive graphics and set pieces, but again, a simpleton hack n' slash style (game isn't as easy as El Shaddai though) and stiff movement with Kratos that leads to much annoying deaths. The story looks nice, though, so I may get to beat this game one day.

Halo: Reach. Almost everyone agrees that Halo: Reach is a masterpiece. It is not. It is the most derivative, dull FPS from the Halo franchise. The story was "good", but it was presented like crap to the player, no connection with the characters (seriously, they're cookie_cutter characters!), big plot holes, not a good soundtrack like in other games and cheap difficulty...what's here to love?

For the record, I never played it online, so my experience wasn't complete. But still, campaign is atrocious, and made me lost all love that was left for the Halo franchise.

The Fire Emblem DS game. A remake of Marth's adventures turned into a putrid artstyle, bad animation, no connection with the characters whatsoever (I mean, I didn't play the original, but whenever someone died no one else cared?) and easy for Fire Emblem standards (though you could pretty much put Lunatic at the beginning and that's pretty lunatic). Seriously what the heck happened here. I just beat two chapters and never looked back again.

Dishonored: I'll never understand myself with this one. This game has all the elements of a great game, yet it bore me to tears. I've tried playing it seven times, only to give up all those. Definitively not a game for me, though I loved Thief and Deus Ex (all games of the franchise), so this comes as a surprise to me. Certainly a disappointment, but not the game's fault this time.

Spelunky: Dunno why this game got so much recognition, but I didn't like it one bit. The fact that a monster comes and kill you if you take too much time kills the enjoyement even more. WTF.

And the music was certainly annoying.

Viva Piñata: I always had high hopes of loving this game. I didn't. Again, not the game's fault, but certainly a game that is not for me. I did love Kameo, though, so at least there's a Rare game I've managed to enjoy in the seventh gen.

cod ghost
gta4
re5 and 6
mgs4
every NFS after mostwanted from 2005

Infamous
GTA IV
Star Ocean: The Last Hope
Final Fantasy XIII
Lair
Killzone: Shadow Fall
Modern Warfare 2
Modern Warfare 3
Spider-Man 3
Genji: Days of the Blade
Street Fighter IV
Mass Effect 3
Assassin's Creed
Assassin's Creed: Revelations
Assassin's Creed 3
Dragon Age 2

Prince of Persia warrior within
Prince of Persia the two thrones
Prince of Persia 2008
Prince of Persia the forgotten sands

because The sands of time was a masterpiece and one of my favorite games and those games are not , WW is great but TSoT is much much better
the others were average at best

Sounds like so many other opinions I hear and usually the games that are disappointing to people are sequels that change too much.  I've played many many many TPS and ME2 and 3 felt very different from all those.  The others I'm uninterested in but the fact that everything is a sequel is quite telling.

battlefield 4
battlefield 3
spelunky
skyrim
total war rome 2(bigges dissapointment)
far cry 1
gta 3
super mario bros 2
more...

Games I came into with high expectations and didn't live up to them-

Metal Gear Solid 4 (PS3): a huge step down from the amazing Metal Gear Solid 3. I just felt like it was half the game. Reviews of the day did nothing but stoke my excitement and when the final credits rolled, I was ready for the game to end. It's the only MGS game that I've only played through one time.

Final Fantasy XIII (PS3): Fun battle system and amazing graphics at the time. Unfortunately, the story was kinda terrible and there was no sense of exploring a huge world. This game made me feel hollow inside. I still bought XIII-2 (never finished) and Lightning Returns (Day one--haven't opened it yet!). Hoping for that old FF feeling to return...but I'm playing X/X-2 in the meantime.

The Last of Us (PS3): I haven't finished it yet (got it in November) so my opinion may change by the end of the playthrough. After the best intro ever, the game hit a brick wall for me. I've been avoiding spoilers for a year now because I still expect it to be great but I shouldn't have to force myself to play a game if it's really great, should I? Another example of the reviews/players hyping a game and fueling my disappointment.

Grand Theft Auto 4 (360): Now I LOVED GTA3 and all of its iterations (Vice City/San Andreas) and I LOVE GTA V. GTA was the black sheep. Maybe it was the lack of variety of missions. Maybe they tried to cram too much in there and it was more annoying than well implemented. I wasn't a fan of this one.

Killer Instinct (XBO): I grew up standing slack-jawed in the arcades watching others play Killer Instinct. After Street Fighter, Killer Instinct was MY fighting game! When I finally played it (It was intimidating to play a game that looked that good. People always gathered around) it played just as good as it looked. The feeling expanded to KI home version on the SNES, KI2 in the Arcade, and KI Gold on the N64. As the recent owner of an Xbox One, I had high hopes for the game and downloaded the free version. I wasn't impressed. Gonna log in some more time and hope it grows on me but the bare bones game we have now just doesn't inspire me to play it.