Exactly. It seemed like the people who liked X-Wing the most were the ones who weren't really that into Wing Commander. The two games represented very different approaches to the same genre: the flashy, movie-like game versus the drier, more "simulation" style game.
Considering X-Wing was based on a movie, I guess I expected it to be even more movie-like than Wing Commander! How ironic that it was less...

Gothic 3 - It was unplayable when it was released and never reached the level of greatness of it's predecessors.
Fallout 3 - great graphics, nice music, fantastic world, all ruined by terrible script, dumb characters and awful parody of the SPECIAL system.
Commandos 3-4 - not even half as great as the 1&2

Oblivion - never felt so let down by a game, the auto-levelling world and never-ending oblivion gate quests just killed it. No amount of pretty scenery could compensate for the boredom and the fact that it's an rpg trying it's hardest to make character development pointless.
Also - the truly mental AI.
I can't think of 2 more, in my head "game" + "disappointment" = Oblivion
- I did play Fallout 3 for a few hours before the tedium and combat system made me put it on ebay, but because of Oblivion my expectations were low anyway.

By Commandos 4 you mean Commandos Strike Force, or there was a 4th part I know nothing about? :S

Hmm, that wasn't quite my experience. Granted, nothing in your starting vault is that much of a threat to you, but I got myself into some severe trouble as soon as I stepped out of it.
There was that abandoned school right by the Vault, which I headed to first before I ever approached Megaton. Raiders were shooting at me from it and I didn't much appreciate that...and it turned out there were more than I expected.
Also, on the first real quest I undertook, I was crouched behind a supermarket counter realizing that I could not aim for crap, had almost no ammo, and was outnumbered. I certainly didn't feel like a demigod there.
Anyway...
Disappointing games!
Let's see.
Lords of the Realm III: I remember when this first came out. I knew very little about it, but I had loved LotR2. And I discovered it was an entirely different game. Only connected by setting and a name.
Master of Orion III: I caught on to MOO2 a little late, but regardless I loved it. I was thrilled to be getting a chance to play a sequel to a game that had done so much right. And then it was revealed to be a macro management game, more about giving up control to your governors than about controlling things yourself.
Civilization IV: By now you might be noticing that strategy games are all my disappointments...I suppose it is because I allow myself to buy into their hype and promise the most. Anyway, I loved the first thee Civ games, and Alpha Centuari. But the fourth game just underwhelmed me. Between a buggy initial release, a much slower paced early period, and the relatively uninteresting impact of religion on the game, I felt like I was holding a prettier but more flawed version of the other games. It shook my faith in Sid Meier a bit, and made me wonder just how big Brian Reynolds had been in the game-making process (and mourn his apparent preference of real-time strategy over turn-based).

Combat!? Don't get me wrong, I love Fallouts, but how could you possibly actually like it's combat? AI was pretty dumb, and at late stages it was all about 'aim at eyes - shoot'. .. Or did you just consider rest of the game so shitty it was the best part of it? :D

Tony Hawk:Underground
Turned an incredibly very fun sports game into a RPG where you can be a virtual jerk! Actually THPS4 started down this path, but this one I needed to trade in, I just couldn't stand it anymore.
NFS:Underground
Actually, kinda halfway fun, the drifting was great. Again, though, the whole RPG part of it really started to get annoying, and introduced a whole series of "barbie for boys" racing games. Bleak years indeed.
Age of Empires III:Underground
(Heh just kidding). It was worth playing through the campaign, but I really expected a LOT more from it. I had already played what I originally thought was the successor to AoE2 - Rise of Nations - and expected this to be much more advanced than that... instead they based the whole thing around a deck of cards? Again, your reward for playing is that you can bling out your city. whee.
And for bonus points:
Uru
Doesn't suck me in like any of the Myst games did. I think it's the third-person thing, which is not only completely unnecessary but is kind of a shocking contrast between the fantastic world in front of you and the mundane one that is represented by you. I still haven't played it for much more than a couple of hours total.

My top 3: Oblivion, Jade Empire, and an old game called Rise of the Robots.

Hmm. For me:
1) Wrestlemania 21. They hyped it up so much, and it was a big MESS.
2) Deus Ex. Held by so many as the ultimate game, for me the terrible controls and archaic engine render it unplayable. Likely a victim of my "has to be played initially while it's current tech" theory.
3) Call of Duty 5. Yuck. Complete step backwards, and tried (Successfully) to get console users to pay $50 for a re-skinned mod.

I'd like to add Halo 3 & ODST
Despite the deliberate cliffhanger, Halo 2's campaign was phenomenal, and the weapons added (like the Battle Rifle, Carbine, Brute Shot, etc; dual-wielding!) added an entirely new dynamic to the game that made it more fun and even more interesting than Halo 1.
And then there's Halo 3. The campaign was too short, we were "married" to Cortana so she took it upon herself to get lost in some Zerg labyrinth and make us find her (the honeymoon was over LOL). The Arbiter was no longer playable (I LOVED playing as the Arbiter), and we revisited what looked like the first Halo, which was actually annihilated. The multiplayer wasn't as cool as Halo 2, even with added items and more turrets and vehicles. None of them felt necessary. I still only use the vehicles from the first and second games. Nothing really has changed from H2 to H3.
Halo ODST: For $30 I wouldnt hate this expansion so much, but they put a full-game price on what is essentially an expansion, and justified it with DLC that hardcore Halo fans already have (All aboard the Failboat!)! The game is essentially Halo 1 with Halo 3 AI, taking on a more tactical approach, but then.....THEY FORGOT THE BATTLE RIFLE. And rarely do you get the Assault Rifle. How am I supposed to kill a Brute with an SMG???
The campaign is an interesting story, but it clocks in at only 6 hours and is rife with cliches that are not creatively implemented. Find the secret scientist who knows about the enemy and escort him out before New Mombasa gets consumed in the slipspace jump. That's it.

SW: Knights of the Old Republic 2 - I adored the first one like crazy, and this was the first game I preordered because I thought it could only be better. Instead, I found it overly complicated, too unfinished to make sense, and I really hated the blatent matchmaking with the NPCs (I wanted the Handmaiden dammit!).
Maybe I was just young and stupid, or didn't pay enough attention, because I've replayed it recently, and I actually enjoy it.
But other than that, nothing. I'm not easy to dissapoint, I suppose.

1 - Overclocked
2 - Overclocked
3 - Overclocked
Yes, it really was/is that bad.

HOMM V
Boring. Just boring.
Lionheart
Need I say more ? Starts great ? No : GREAT. And then...
Star Control 3
Not playable. SC 2 was SO good...

Man I'd forgotten Rise of the Robots, got in on release day and even as young as I was I very quickly realised it was garbage.