The most frustrating thing about Brawl's netcode is that they got the online 100% right with Mario Kart Wii.  I know, different teams and all that, but still under the Nintendo umbrella.

I didn't like Mario Kart Wii.  It felt like a big step down after Double Dash (much like my feelings about Melee -> Brawl).  But I don't list that among "most disappointing games" because the online was a big step forward.  I loved it.  Can't say the same for Brawl so it gets relagated to these kind of lists for me.

assasins creed 3

Mass effect 3 deserves an honorable mention

Most games i was hyped for end up being brilliant. My hype has good taste.

Original Final Fantasy XIV, and also to a smaller lesser extent A Realm Reborn.

Final Fantasy 13.

Fallout: New Vegas.

Assassins Creed 2.

Tomb Raider.

All of these games disappointed me

Final Fantasy XIII is really the poster child for this thread. A reviewer I follow said "XIII isn't bad or even mediocre, but it is disappointing."

Resident Evil 6. WTF Capcom. Worst "AAA" game released by a fair bit this generation. In a lot of ways, this is the polar opposite of XIII. XIII largely failed because it tried to be awesome. RE6 failed because it was a just plain bad game. 

3rd Birthday. The game which disappointed me personally the most. It ruined the legacy of an otherwise perfectly good action horror series. Most fan-fiction is better conceived.

Agreed.  XIII-2 was basically the best parts of Chrono Trigger, FF VIII, and Pokemon all wrapped up into one. It's got faults, sure--Ali Hillis cannot read a Shakespears-like line to save her life, and it shows because Liam O'Brien is incredible at them--but it's still bloody awesome. And I LOVE the ending.

It could be that I'm alone in the world, but I really enjoyed Other M and liked the design of Samus a lot more than in Prime.

FF XIII. Many people on this forum will remember just how hyped I was, and many will agree what a piece of fucking trash the actual "game" is.

GTA IV.