I'll do one for consoles and one for handhelds:
Metroid: Other M (Wii): At the time Metroid: Other M was announced, I had played the Prime series, Metroid: Zero Mission and Super Metroid. So with such a pedigree and Sakamoto behind the game, I was pretty excited for the title. But when I finally got to play the game, what we got was a lot of bad design choices. 3D action combat controls were awkwardly shoehorned on the Wii Remote (It didn't even give you the option to attach the Nunchuck), 1st person "pixel hunts" which served no purpose other than to cause frustration, Samus Aran's seemingly out of nowhere design/character choices which only served to take her two steps back instead of actually fleshing her out (freaking high heels on her zero suit, her never ending quest to please/obey the "man in charge", even if it meant self harm), a nonsensical story full of unresolved fluff (The Deleter!) and holes (Adam's personal magic gun which can automatically shut Samus' suit down.), and a final boss which will kill you over and over again unless you realize that you can use the Power Bomb you never knew you had (and the game doesn't tell you that you have it).
All in all, this game halted Samus Aran's incredible momentum leading up to this moment. Not only with poor sales, but with how we've come to know a heroine and the adventures she's taken (the Prime series apparently isn't canon now, thanks to Sakamoto). Just one big disappointment. 
Yoshi's Island DS (DS): Super Mario World 2: Yoshi's Island on the SNES is considered to be one of the greatest games of all time, and for good reason. So when Nintendo announced Yoshi's Island DS, people were excited to see a return of fantastic visuals and unique platforming gameplay.
However, Yoshi's Island DS was just... "good" at best (and uninspired and unfun at worst), and not reminiscent of the platforming perfection we were expecting. Odd difficulty spikes, trial and error level design, and more odd design choices seemed odd for such a game... until we realized that the game wasn't internally developed. Yoshi's Island DS was actually developed by Artoon (Blinx the Time Sweeper for Xbox). So while it may be par for the course for that studio, fans were expecting something much more magical.
What's frustrating is that Nintendo is doing the exact same thing with Yoshi's New Island for the 3DS. Instead of being created by Nintendo's quality studios, the game is being developed by Arzest Studios. What are they known for? Wii Play Motion and StreetPass Mii Plaza. Consider my excitement lowered.