The most disappointed DS games I've played? I dunno, I'm not qualified to judge the emotional state of a video game.
Bad puns aside, the most disappointing DS games I've played have to probably be The World Ends With You and Heroes of Mana.
The World Ends With You I disliked because, as somebody stated earlier, I hated having to control two characters and play a rhythm-based minigame for each, AT THE SAME TIME. EVERY COMBAT. And the AI screws up more than you do, when you let it handle one character. Which is a shame, because I liked the story, and wanted to see more, but gave up a few hours in because the battle system was so poorly designed. I simply couldn't understand why so many people proclaim it to be the BEST DS GAME EVAR! So much hype for so much disappointment.
As a longtime fan of the Seiken Densetsu games (all the way back to Secret of Mana on the SNES; my brothers and I have logged untold hours playing that game), I was disappointed by Heroes of Mana, the shallow RTS that pretended to be part of the series. In this case, though, it's simply that the game isn't really memorable, as opposed to any specific crippling flaws. I liked Children of Mana; I went in with high expectations, only to have them dashed by a mediocre RTS wearing an RPG suit.
Those were my two big DS disappointments. That's not to say that those are the only two DS games I disliked, or even that they are the ones I disliked the most; simply that they were games where I went in with very high expectations, which were not realized.
