In no particular order:

Zack & Wiki: Quest for Barbaros' Treasure (Wii) - Ton of hype here and all around the internet. First 20 levels are great. Last 4 levels blow. They turn the game into a bad action game with terrible controls. Why not just stay a puzzle game?

Cursed Mountain (Wii) - Cool concept. Poor execution. Game only has about 2 great levels that really work well, and they are both more than halfway through the game. Not really scary at all. Every time there is a ghost, it announces its presence with a short cut-scene. I have never played a game where the character moves so slow. Makes sense on the mountain near the end of the game. Makes no sense in the first half.

A Boy and His Blob (Wii) - A fiddly puzzle platformer. Not very satisfying to beat. It got a lot of hype around here and pretty good scores from critics. I had the same reaction to Toki Tori. On the other hand, Fluidity is fucking fantastic.

Final Fantasy Crystal Chronicles (GC) - Barely even Final Fantasy in my opinion. Made it through a few "years" and quit, never to return. Boring story, boring gameplay.

SoulCalibur II (GC) - Hyped as a great fighting game. I thought it was just meh. I think Smash Bros. has permanently ruined other fighting games for me. Smash is just so much better than everything else.

TimeSplitters II (GC) - It was a good game. But after hearing about how it had some of the developers from GoldenEye and Perfect Dark, I was expecting so much more. Just didn't live up to those two games. I was kind of disappointed with FPS on GC in general after they were so great on the N64. 007 Nightfire and XIII didn't really click with me either.

Black & White (PC) - Cool concept, but it just didn't seem work. Terrible controls too.

Command & Conquer (N64) - Loved the PC games. Don't know why I expected this to be good on a console.

Super Ghouls 'n Ghosts (SNES) - I was really young at the time I had this. Just way too hard for a kid. Only game I ever traded in.

Spider-Man 2 (GBA) - Got this game as a gift. The first level is a timed event where you have to deliver pizzas to 3 or 4 different customers. There is no indication on how to find them. Just hope you find them in the maze before time runs out. Never did get past this part. Frankly, based on what I saw, didn't seem worth it to try very hard either.

Have to agree. Though it wasn't as big of a disappointment to me as the games I mentioned because the base game still kicks *ss and I knew what to expect, I'd definitely rather play the original. Every gameplay-change they made was either useless, or a bad one.

Mk8 and Smash Wii U.

Civilization Revolution. I had high hopes for this but the worlds it generated were too small and each playthrough felt like it was over too quick. Resources had no real benefit. This game felt like a graphical remaster of Civ 1 but even that game has more content.

You take that back!

But out of curiosity, why?

Star Fox for WiiU and No man sky disapointed a lot of people
It really depends on how much are you expecting...

This might actually make a good article for the site. I'd probably get death threats though...

Considering it was part the yet-to-be-proven-bad Wii Series, yeah, why not. But, it wasn't even okay...just, man. I wonder if they'll ever come back to it.

MK8? Really? I found it near identical to the Wii one.

As for Smash WiiU, well, smash is such a stylistic fighting game it really depends on your original preference for Smash (64,Melee,Brawl). I enjoyed all three and found it as a nice balance.

In that sense, it would have to be about disappointments in general, rather than one's projection towards it. So the usual suspects, like Devil May Cry 2, Call of Duty Ghosts or the new SimCity.