Don't get me wrong; the DS is a nice machine with a ton of good games. But the phrase killer app makes me think of that "must have" game that I played to death and justified a system for me. The DS didn't really have that for me.
I'm downsizing my systems since I don't have the time to play them all, and I didn't play the DS (and it didn't have a killer app or apps for me), so off it went
I like shmups, action-type platformers (run'n'gun or hack'n'slash is my preference, but traditional platformers like Sonic or Mario are fun as well), strategy games (although I'm pretty picky) and racing games.
Someone mentioned Atlus earlier in the thread; I wanted to get Tokyo Beatdown, as that looked right up my alley. Maybe that would have made me keep it

The reality is that I rarely have my portable game machines on me when I need them (i.e. I find myself stuck waiting in a doctor's office, in line, or waiting for a friend, etc.), except for my trusty iPhone. So I now have a ton of iPhone games and no more handheld systems.