Mass Effect 2, Dragon Age 2, Crysis 2, Assassin's Creed Rev, whatever the last Prince of Persia was, Paper Mario Sticker Star, Metroid Other M, Deus Ex: Mankind Divided (more like gameplay divided)... There's too damn many to list.

Dragon Age 2
Dragon Age Inquisition
Mass Effect 2
Mass Effect 3 (only because of Kai Leng, the Invulnerable Prancing Swordsman(™) and "that ending")
Mass Effect Andromeda (what a horrendous mess!)

There's more, but there always is.

Mafia 2 took the masterpiece that is Mafia 1 ( Mafia 1 is easily one of the top 5 best games ever made, period) and dumbed it down for console gamers & GTA fans (GTA has always been a vastly dumbed down and vastly inferior counterpart to Mafia 1). Mafia 2 is a mediocre game in its own right, but compared to Mafia 1, Mafia 2 is utter trash.

I avoid Mafia 3 like the plague because I can tell it is even much worse than Mafia 2. What a shame.

Final Fantasy 8. I just couldn't enjoy anything about the game. Well, the graphics were nice for PSX. But yeah. I think the best way to play the game is to not win any battles, and that's just silly to me.

How is it humanly possible for someone to have such shit taste?

I have great nostalgia for that game because it was one of the first games I played when I got my very own computer in 2004, but I know I'd never have the patience for it nowadays. :-P Fantastic orchestra rendition of the theme song though, making an already beautiful theme more majestic.

As for disappointing sequels, I dunno, generally I think it's just nostalgia for the originals that made me go "meh" for sequels like LEGO Island 2 or, more recently, Unreal 2. I enjoyed Tony Hawk 3 and 4 less than 2 but I know that they're all good games.

The Witcher 2

Saints Row Gat Out Of Hell

Great concept, extremely disappointing outcome.

Aliens: Colonial Marines (we all know about that)

Divinity: Original Sin (should have been real times combat not god awful turn based badly implemented)

Duke Nukem: Whenever

Grim Dawn.... just not a patch on Titan Quest

Risen 3 (not terrible but very boring as if felt overly long and repetitively trivial after a while.)

Alien Isolation (decent game, but the save feature frustrated me so much I gave up after a few short hours)

Transistor (after Bastion I was expecting a captivating game with rich gameplay. What I got was over-stylish bobbins with a terrible combat system awful storytelling)

Total War: Rome 2 (I started playing total War with he awesome Shogun 2, Rome 2 was awful in comparison)

Heh, I thought the combat was the only thing that's fun and original about it. I'd have preferred if they had cut down on the writing instead.

But I wouldn't really call it a sequel to the former Divinity games any more than I would call Divinity: Dragon Commander a sequel. At least they didn't name it Divinity 3: Original Sin. The gap from Divine Divinity to Divinity 2 was much bigger (even though I kind of liked both of these for what they are).

Yeah, I was heavily invested in the series right from the start, so even when Commandos 3 turned back the dial I was already heavily disappointed. And then they decided to make a multiplatform Commandos game... queue passable fps... series dead...

I'm also a huge Warcraft fan btw, with Warcraft 3 being my alltime favorite rts. At least due to the series' crazy popularity there still is an outside chance of seeing a Warcraft 4? But I'm really worried after SC:WoL which to me told a cliched story that utterly failed to engross me, especially when compared to Warcraft 3's tale and characters.
Though perhaps more worrying is what WoW did and keeps doing to the lore. I don't know what story they could tell with 4. For instance the Arthas story I would've loved to wrap up in a proper rts sequel, but no, that story now concludes in a WoW raid :P What's more, the conclusion of Arthas' epic tale suffered the wrath of MMO story telling [spoiler alert for what it's worth - from wowwiki] :

"His (Arthas) plan nearly succeeded when a group of adventurers led by Tirion Fordring finally ascended to the Frozen Throne and did battle with him after Horde, Alliance and Ashen Verdict forces assaulted Icecrown Citadel"

However you must forgive me :) I will contend that WoW is a good game, especially the vanilla zones. I only spent about 5 months in the game post Wrath of the Lich King, but my time in the vanilla zones especially created some unforgettable moments. They really managed to capture the Warcraft feel (again, in the vanilla zones). Strolling through Elwynn Forest for the first time with this tune marks one of my favorite moments in gaming ever.

I have by now completely lost what little grip I ever had on Warcraft's lore. I still sometimes read the news about the new expansions for WoW, but when I see they now had time travel and alternate dimensions I just roll my eyes and don't even try to figure this shit out.

Mind you, I never said it's a terrible game, just how much it disappointed me. I guess it's an ok game, if one likes the genre. I find the whole "MMO paradigm" that Warcraft is the pinnacle of to be just tedious and boring. But I know that there's plenty of people who like it, and I'll admit that for what it is, WoW is (ar at least was, waaaay back when I tried playing it) technically solid. It's just that for me that "what it is" is a disappointment in itself.

Ok, so, fool me once, shame on you. Fool me twice, shame on me. Fool me five times...!? ;) :)

Ultima 9 !
Previously I thought Ultima 8 was a huge letdown after the two amazing Ultima 7 parts and I thought, Ultima 9 couldn't possibly be worse. But boy, was I wrong! It's as if someone intentionally trashed everything that made Ultima great.

MOO3 was also a huge disappointment, like others have already mentioned.

Fallout 4.
I loved Fallout 3 and NV and couldn't wait for the sequel. The game just feels soulless to me. It is just go here, kill them. Go there, kill them. The quests and locations needed more variety and love put in to them (there were a few good ones).
I don't hate the game but it is a bit meh.