This is a nice thread :)

As for the game, I would say Dungeons....seriously....*insert curse words here*

Hellgate:London and Spore come to mind as the games I was most looking forward to and that turned out to be big dissapointments.
They are not the most terrible games I ever played,but in terms of potential they were the most dissapointing. BOth were games by designers I had enormous respect for, and in both cases the designers failed to live up to their reputation.
Dungeons from day one I suspected to be a lame pretender to the DK throne, SInce none of the people responsible for the DK games were involved, I decided to wait and see what the reviews and WOM would be like, and thereby saved myself Forty bucks.

I'm one of those who usually blissfully avoids the hype and doesn't make many expectations of games, especially sequels. When I said Fallout 3 was a let down, it's actually saying something because I wasn't expecting that much from it (what finally broke it for me and made me say "Fuck this, it isn't Fallout at all." is when you hack what's-his-names computer in Megaton and find out that douchebag raped generic victim number 7, you couldn't do anything about it.

Other letdowns for me, AoE 3:
Too many of the modern stylings in RTS, arbitrary caps on buildings, low unit caps, etc, and generally just bleah.

Warhammer 40K: Soulstorm:
Dark Crusade is in my top 5 RTS, Soulstorm was still good (I loved the Sisters of Battle), but a couple key features were missing in it for me.

Sanctum:
Even most free flash games have more content than Sanctum does, and while it's a cool concept (merging a FPs and a Tower Defense), as a FPS it was weak (no major flaws, but nothing to stand out) and as a TD it was flat out generic. Doesn't even have an attempt at a story either.

The I of the Dragon:
To be fair, I paid like $2.50 for this one, but still.. it's pretty hard to completely ruin a Diablo Clone, yet this manages it, the controls are finicky, the action is.. not. all it has going for it is "Dude, I play a dragon! Rawr!"

Hellgate: London:
"It's like a postapocalypse Diablo with guns!" .. I love Diablo, love post-apocalypse and am a gun nut.. yet.. while it was entertaining, it failed to stand out as much as I expected.

Final Fantasy 8:
I came off of FF7 (the first FF i'd played), and I was expecting something equally awesome. Instead I got this whiney bitch I wanted to slap, a bunch of other characters that faded in to the background (Zell and .. Irvine? (the snipers) were cool though), and a plot that left me going "What.. the.. hell?" Pair that with their bullshit idea of level scaling, and the game actually being easier if you barely level up at all, as opposed to you grind levels (like everyone does in JRPGs). Seriously, in any RPG, iif you have a hard time with a fight, and keep having a hard time after you rethink your tactics, you go level up a few more times to get new abilities.. you don't expect the fight to be even harder when you get back.

Fixed. :p

I liked AoE 3, but then again it cost me less than the credit card fee to buy, so it would be hard for me not to get my money's worth out of that one.

Don't get me wrong, I like it.. I was just expecting further improvement on AoE2. :)

Oh yeah me too. Had lots of expectations for it, but never made it past the 4th level or so. Villagers are too annoying, and can't do anything right. The pets are cool though.

Neverwinter Nights. After BG1 and 2 and I was expecting another RPG of the same caliber, with heavily developed characters, and a solid, well written, story. Instead it was an incredibly bland exercise in repetition. It was almost completely devoid of any of the atmosphere and fantastical environments (yes, fantasy but not fantastic) that made the Sword Coast so interesting. I pretty much see this as the point where Bioware gave on writing, and just began pumping out one generic RPG after another. I've disliked every one of their games after it. Though I did love NWN2 and KotOR 2, because Obsidian were able to salvage the both of those.

Oblivion.

Biggest disappointment ever. Not even a fraction the game Morrowind was. Oh well.

In the "sorta" category, we have... the Witcher 2. Given how much I enjoyed the first game, I was basically expecting "Jesus on Ice" and while what I got was good, what I really wanted was more of the Witcher 1. Of course, they haven't finished patching TW2 yet, and TW1 was a much better game after its patch cycle finished, so there's some hope for this yet.

In the "absolutely" category, we get Spore, DA2, and Daikatana.

I've noticed that most of these have to do, simply, with ridiculous expectations. Many of the games listed in this thread aren't bad, they just didn't live up to the enormous hype they got before release. If they were marketed as "a solid new game in the X genre" instead of "God's Gift to X" I bet they wouldn't have inspired the hate.

I had real problems with the campaign in AoE 3. I very much preferred the way they did the campaigns in the first two AOE games, with a number of campaigns focused on real historical figures and events. I did not really care for the single campaign in AoE3, built around a fictional family. That took away a lot of the historical flavor the first two games had, and since History is a main reason I play these games,that was important to me.
With Spore, it was hyped as a new game experience, unlike anything you had played before and it ended up being pretty much a Civilzation style game that morphed into a MAsters of Orion style Space conquest game, and not being very good at either.

Mass Effect 2.

It dissapointed me so much, that I've pretty much given up on BioWare for now.

Throw in "The Movies" to that mix.
I always wanted a FIlm Studio Sim, and The Movies was not terrible but not what it could have been. I would have preferred some more realism and less emphasis on that "Movie Making" program included with the game. It was too limited to tell you much or give you a feel for a studio editing room, but was just complex enough to be a distraction from the main game.
Not a bad game, but not what it could have been.

My conclusion on Spore:
They released Spore to pay for the development of the creature creator they used for DarkSpore ;)