the 360 failure rate is my biggest disappointment.

None. All the games & systems I have bought have at least matched my expectations.
I love not being picky. :)

Perfect Dark Zero should be on the most disappointing list.

The constant delays to UK releases of big games.

 The single player GP's aren't cruel at all. They take skill.  Races on 150cc (and lower) use to have me neck and neck with the computer (or at least gave me some competition), but as you progress through the game you should be getting better and/or smarter with your item management and knowledge of the courses.  I can put enough distance behind me and the computer within a lap to easily take a blue shell.  By the third lap I can take four before it's a close race.  You can also tell the idiots who didn't even give the game much time, because they assume the POW block always fucks you - sorry, but I love them 99% of the time because most players don't realize you can avoid them with a simple shake of the wheel.

While Mario Kart Wii could be frustrating at times, it is fun as hell and you really do see a real progression in your skill from when you start till when you've unlocked everything.  IMO, any reviewer who hasn't unlocked everything or nearly everything (~30-50 hours of gameplay depending on how quickly you improve and whether you go online much) hasn't played the game long enough to offer a fair opinion.

http://kotaku.com/gaming/mark-rein/gears-of-war-a-mere-10-million-to-make-234504.php

 Its becasue people are retarded, they see one reviewer give a low mark so they all do.  The same thing can be said for GTA4 with its high marks all over the place.  I have played the Oragne box on the PS3 and there are under 5 slowdowns during all the half lifes and they don't last very long.  The loading times are not that very long and they don't happen very much.  Portal and TF2 ran fine aswell.

 Common knowledge?  Thats why no one who owns the game ever had these massive problems that game reviewers say they had.  Its also common knowledge that they wii is two GC's duct taped together.  And wii Mario Kart looks worse then DD mario Kart.

MK Wii is not a disappointment. It's only one to people that aren't willing to put in the time it takes to become good at it. Assassin's Creed also wouldn't be considered a disappointment to me. Repetitive but very cool. The other ones you mentioned definitely are. I think that you can add Haze to your list.

dissapointments of this generation = still high software prices in the uk