Oblivion spit in the face of Morrowind. I still really enjoyed Oblivion but it could never live up to Morrowind in my book.

Fable 3. I love the first Fable game and enjoyed the second one. I was really hoping Fable three would combine the first 2 together and make a great game; I was wrong dead wrong.

Neverwinter Nights. I played it for an hour or so, then uninstalled it and sold it to someone. In those days I was just too lazy to take some time to get into the story, the characters and so on.

Today I truely regret that I didn't and I wanted to give this game a second chance when I saw it here on GOG. But until today I didn't buy it because I actually run low on money as a student at the moment. So who knows... maybe in another 10 years this title will get another chance. It would be the third then, I guess... :)

Portal - of course overhyped (by forum people). The concept was pretty awesome to me, and the puzzles were clever, but as a whole the game didn't bowl me over. And the end song - catchy and fun ... that's about it.

Neverwinter Nights 2 - I did finish this game, but was just disappointed overall with the gameplay. It also ran very slowly during location transitions on the PC I owned at the time. I also remember being really disappointed at the small number of locations, like stores, etc., on the maps of that game. I was, and still am, a huge NWN 1 fan. Maybe I should try it again after reading here about its improvements.

Fallout New Vegas - tried a couple of times to get into it, but it just doesn't grab me. Loved all the others (actually, just playing Fallout 2 for the first time after downloading from GOG).

Sims 3 - don't shoot me for bringing this up, I never see any mention of the Sims on this site and think it's probably sneered at in general, but I have spent many happy hours with the Sims 1 & 2. Sims 3, in spite of having many upgrades in terms of the persistent world and character creation, just. is. not. fun. Plus, it is the buggiest thing I've every played, probably because of the complexity of the code - but they rarely fix any of the game-busting bugs before releasing yet another add-on. Greedy bastards.

My biggest letdowns were:

Quake 4
Daikatana
Dragon Age 1
Final Fantasy 13
Final Fantasy 14

Nah, if its no fun, move on. Personally I would put Morrowind easily in the top 10 games of all time category, but everyone has a preference. Do what is fun for you. You mentioned mods. Oblivion would have been very disappointing to me but with the great mod tools at release, I was able to fix just about everything they did that I didn't like. And since they released the tools as they did, and made a decent core game, I'm loving Oblivion as well, but just not as much.

To add a second game to the list for me...

Auryn's Quest. To be fair, there was no hype, I didn't anticipate anything... but this was still the worst piece of junk I have ever played. It is supposed to be part of the Neverending Story story. It isn't. Its just 12 textures splashed on ridiculously uninteresting and in-congruent levels that ultimately lead to a disco tech rolling puzzle sim that takes about 45 minutes to beat the entire game twice. (yes, twice, we thought we did something wrong).

I paid $1.90 from a Gogamer 48 hour madness sale and literally felt ripped off. Years later I'm still contemplating emotional damages + my $1.90 back :p

Worst. Game. Ever.

Black and white 2. Years of waiting, bought collectors edition. It was a complete mess, and they knew it when they released it. Lionhead totally sold out.

Exactly my opinion! Finally someone who agrees with me on that. :p

And as for my dissapointments:
Deus Ex 2. Only played the demo, but that experience plus the negative reviews from respected gaming magazines made me never buy it.

Half-Life 2. I played through this with a strong meh feeling. The focus on vehicles, generic outdoors environments and "I'm a rebel teenager and the police are evil" story are my biggest problems with the game.

Dragon Age. It sucked balls.

And umm, Daikatana I guess. :p Can't really think of any more right now but I'm sure there are loads of them.

You really had that with HL2? When did you play the game for the first time?

Ok, last one, I swear. Halo. I was fairly disdainful of it, until I played Marathon (which, by the way, I refuse to believe it's not set in the same universe), and read The Fall of Reach, and loved the world that was built up, so I figured, sure, I'll get it. So I got it (PC version, of course), and actually quite enjoyed it. For the first half. But the second The Flood showed up, it suddenly got crappy. The enemies weren't interesting, the levels went from fairly wide and open to essentially corridors, and the whole 'disable the generators by running into them, losing your shield, being blinded and swarmed by enemies' was a horrible design choice. Of course the ones I would love, ODST and Reach, aren't coming to PC.

Civilization V and Age of Empires III are biggest disappointments for me. Anything Valve has developed or published has disappointed without exception. Final Fantasy series as a whole was also disappointing for me.

Most of my gaming disappointments have come from me listening to the hype, or letting a review do the work of bigging it up for me.

The PCG UK review of BioShock, for example, made it sound like EXACTLY the sort of game I had an itch for. I got about half-way through it before I gave up. So much about it was crap, I have a hard time seeing why more people didn't call it so on release. I've tried to go back to it a few times over the years. On my last attempt, I couldn't get passed the opening thirty minutes.

Also with "Thief: Deadly Shadows". Primarily The Cradle. Again, I listened to everyone raving about how awesome and terrifying a level it was. "Great!", I think. "I haven't been even remotely unnerved by a game since 'Vampire Bloodlines', nor afraid of the dark since 'AvP'! I'll have me some of that!" Got to The Cradle. Completed The Cradle. Couldn't be bothered to carry on afterwards due to being completely underwhelmed by it. Uninstalled. Never played it since.

The only game I can think of that hasn't lived up to my own personal expectations is StarCraft 2. Mostly this is due to having the Battle.net-drm bite me in the arse. But also I think it's because of how the story is presented. The problem with 'choose your own mission' systems like in SC2 is that it robs the narrative of any urgency. "Quick! We need help! We can't hold out any longer!" * does five other missions spanning a month of in-game time * Not that urgent, then.

Half-Life 2, Deus Ex IW, Bioshock, Mafia (yes, the Mafia 1), Mass Effect, Twitcher 1 (yes, THE Witcher 1), to name a few amongst many others.

Only games I can remember having bought thinking I would enjoy them, and then been severely disappointed in would be, at least of the top of my head, Phoenix Wright: Ace Attorney and Final Fantasy XII.

I was disappointed in Hellgate: London, as I was looking forward to that prior to it's release, but by the time I bought it I already knew it was going to be disappointing.