Just Cause 2. I thought that, since the first one was so beautiful and fun, yet so horrible and broken, that the second one would be the perfected version. The previews and demo made it out to be just that. I bought it new for 60 bucks and kinda regretted it when it was over. It was great fun and all but, the absolutely brilliant stuff that was in the first game, either didn't show up or wasn't as good. The music? Not stylish at all. The island? Somehow it was more boring with too many environments. They focused too much on variety and the landscapes showed that. The first game is my favorite exploration game ever, second one was just ok. Grappling hook kinda ruined the scale too.

Fallut: New Vegas. I loved all of the other Fallout games, but for some reason this one just didn't really do it for me. I couldn't really get into it.

I know what you mean, although I would give it a 7/10 myself.
It's fun, but really it's just a very fancy Phoenix Wright.
A lot of games have disappointed me in the past, not just DNF ;)
The problem is usually 'overhyping', no game can ever live up to the hype surrounding it.

All I can say right now is Fallout 3.

I didn't expect much, but I still expected better than an underpolished Oblivion mod that lacked any trace of what made Fallout so great. >.>

Dragon Age 2. But I know that's very much my own fault for more or less being in denial about all the changes I knew they were making, and creating false hope that the game would be closer to its predecessor than it realistically could be. I loved Dragon Age: Origins and Awakening and really wanted to see a worthy sequel that would take the original and just make improvements where necessary (like Baldur's Gate 2 compared to BG1), not a complete overhaul of the gameplay systems and the setting. Most disappointing game and most baffling franchise turnaround ever.

Another one was Neverwinter Nights 2. Though my disappointment with that one can be almost completely attributed to technical issues. I had followed NWN2's development very closely and had a LOT to look forward to: they took almost every aspect of the original that I felt was a flaw or could be improved and replaced it or made it better. But when the game was released it was so buggy that I couldn't for the life of me get it to run properly on my system, which was well beyond the recommended requirements. Over the years it got better of course, and in its current state NWN2 is one of my favorite RPG's of all time, but that initial disappointment stung pretty hard.

Being a huge fan of Morrowind and then playing Oblivion was probably the biggest letdown ever in my gaming history. That game is garbage.

Likewise with the Gothics. I love those games to bits, even III. Obviously loved Risen too. I was really out of date with the developers and one day I saw on Steam that Arcania was going to be released that night. Stayed up until midnight and bought it full price. Epic, epic letdown. That game is poo. It was obviously a disappointment, but after my Morrowind-Oblivion saga I already knew what to expect from Bethesda and FO3 so avoided it like the plague. Best 40 pounds I ever saved.

Syberia. The only buy on GOG that I slightly regret...
I love adventure games, played through almost all Sierra Quest games, Lucasfilm adventures, Longest Journey, Kyrandia and others. Thought I would like Syberia too, but I really had to force myself playing it till the end. Too slow, not enough likeable characters, boring puzzles and being forced to endlessly watch the heroine walk, run, climb up and down stairs through dozens of nicely rendered, but almost empty screens with almost no interactive objects or people to talk to. That game felt somehow sterile and was more work than fun for me.

Also Mafia 2 and Crysis 2. Both disappointing sequels to great games.

I would have said LA Noire as well, the most recent example I can think of. Too many underwhelming GTA-isms distract it from the unique concepts that could have been better fleshed out.

Dragon Age. Still enjoyable to an extent, but I'm scratching my head wondering what justifies it as being spiritual successor to Baldur's Gate other than being a party-based fantasy RPG with some dialogue choices.

Heroes of Might and Magic V was the last game that disappointed me because it failed to innovate on any meaningful level (before that, it was Master of Orion 3 because it was mediocre).

After that, I stopped viewing sequels as a logical extensions to one another and choose to look at each game individually on it's own individual merit (taking the prequels into account to the extent of determining if the latest sequel added anything to the table).

I now withhold any sort of expectation from big names until the game is out, though I'll still let myself be somewhat biased by the developers if they have a good track record.

Alone in the Dark 2008. I'd seen the reviews, and waited 'till it was down to about 20 to 30 bucks (I can't really remember). I got it expecting to have something that was a little unpolished with some nice touches with a few glitches. Instead I got crappy controls, a shitty fast forward button (I skipped like 3 chapters. Each chapter had the exact same 'Last time on Alone in the Dark' segment, until the third one, when all of the sudden, it's the same except for a little bit tacked on at the end where, BAM! Satan himself is involved! Spoiler alert, I guess), a horrible inventory system, awful combat (partly due to controls, as above) and stupid platforming and puzzles. I really, really wanted to like it, I eagerly awaited a patch, but one never came. T_T

Also, Dark Salvation. I hear it's an indie game, made with the iD Tech 3 engine, and got good reviews with reviewers comparing it to games like Blood and Painkiller. So I see on their website that they're having a give away contest for the disc edition, so I enter and win. I get it, install and I am greeted with... a fucking awful game. I have had it for about a year and I am still not past the first level. First room, a crystal and a locked door. Break the crystal, there's a switch to open the door... but if you go the way they seem to intend you to go, it's practically impossible to get to the door before it closes, since every door is on a fucking timer. Past that, a room with an enemy hiding in the rafters, who will hit you unless you know where he is ahead of time. Also, a jumping puzzle. Two pillars moving up and down, and a platform going from one pillar to a switch, to open a door by the other pillar. Timed, of course. Beyond that is place that's actually not too bad, there's some annoying enemies, but you can see that the combat is pretty good. Flip some switched, head up, and you find a single door that's not on a timer. Beyond it is another jumping puzzle. Did I mention that all these jumping puzzles have lava? And if you die you get sent back to the beginning of the level? And that when asked why, the developers said it was because "We're trying to make a throwback to older FPS's"? And that no old FPS I have ever played had done that? And that I played a single player mod for Quake 3 that has the same issue? Meaning that they were just being lazy fucks who tried to cover their laziness with lies. Beyond that room is a room with a couple enemies, nothing too bad. Get a new weapon that's pretty much a reskinned lightning gun from Q3A. Then you have to go back to the pillar room, where there is a door that's opened, and beyond that is more level. With more times switches, and jumping puzzles, and a switch you have to hit that's guarded by things that will shove you off into lava. Get through that and there's a room with a new enemy, a crystal, and a fucking deathtrap that will kill you. That's the furthest I've gotten. And those crystals? There's 10 of them. And you have to break them all to exit the level. It's possible to forget the first one, and as far as I know, when that door closes behind you, you can't get back. The game is set in Hell, but I think the game is Hell, and when I die, I will slowly awake, and see the pillar room. And I will scream that eternal, soul searing scream of the damned for all eternity.
Also, they released the game for free now. I have to tell people that. So others can share in my pain. And maybe, just maybe, they can get past level 1, and send me a save file.

I am a big fan of Carmageddon 1-2. I still have Car2 installed on my laptop, still playing it.

Thus, I was very interested in Burnout games (was it Burnout 3), but in the end it wasn't so good I had imagined and read in articles. Quite boring actually.

I was also hoping that Twisted Metal games would be like Carmageddon, but sadly they were not. I played Twisted Metal 2 on PC, and it felt like a simple children's game compared to Carmageddon series.

Empire Total War: 'We will put modding tools out for you guys.' Game releases in a broken state and the developers basically retract their statement leaving the community with no tools because they want to sell DLC. I have yet to buy another Total War game because of this horrible launch.

Neverwinter Nights 2: I loved the original. I bought it the day they removed the DRM expecting amazing things because it was Obsidian/Troika/Black Isle. Well.. The game itself was good except for the camera. I've never had my interest so completely killed in a game by a simple feature. Tried getting back into it recently and its just unplayable.

Guild Wars Factions: I don't do PvP so this expansion and its laughable story really turned me off. The Ritualist was pretty cool though, sort of sad I deleted mine to make room for something else.

Dragon Age Origins: The game was a pretty cool throwback to Baldur's Gate. Hell I even thought it was pretty good despite all the glaring faults... Until I entered the camp and found a guy offering a quest. Being an RPG I take a quest and find I have to buy DLC. On day 1. Pissed off doesn't describe how I felt at seeing this and I refuse to buy another Bioware product because of this.

Hellgate London: Do I even need to explain this? At least the relaunched one is what the original should have been. Well expect for the laughable translation.

I could go on but I'll keep it at 5 for the sake of not clogging up the thread.

Oblivion, Mafia 2, Dragon Age 2. Many companies should take some hints from CD Projekt as to how a good sequel is made.

Oh, and how could I forget it!

Diablo 2 (+expansion). I am currently playing it, in the second act now, and I just can't understand what made it the phenomenon it was, at least single-player. Far too repetitive game for me, and new skills are mostly quite uninteresting, as are the new items you find. I'm not quite sure why I don't have the same problem with e.g. Baldur's Gate.

Also Warcraft 3 (+expansion) was quite a letdown. I simply love Starcraft + Brood Wars and have played the single-player campaign through several times, and I love the story.

The story in WC3 was rather uninteresting in comparison, gameplay was too fast even in slowest setting, and the 3D graphics didn't work that well IMHO (in e.g. Age of Mythology, they worked great). The game area in WC3 always felt too cramped and looked odd, bright colors everywhere and not clear whether some part of land is accessible or not. Even the different factions that you can play are not as interesting as in Starcraft.

I would also say that Total Annihilation games were much overhyped IMHO. Having dozens of almost identical units at your disposal does not a good game make.