I think the PlayStation fanbase doesn't get enough consideration when it comes to the franchises that Sony has brought to the gaming industry since they released the original PlayStation.
No, Sony may not have a singular title that has the iconic status of a Mario or Zelda, but look at their stable of franchises that are immensely popular, even if they don't all sell 8 million units:
Twisted Metal
Gran Turismo (they're most successful franchise)
wipEout
Uncharted
inFAMOUS
God of War
Ape Escape (that series is still going strong, surprisingly enough)
Hot Shots/Everybody's Golf
Jak and Daxter
Sly Cooper
Ratchet and Clank
LittleBigPlanet
Motorstorm
Yes, they aren't all on the same level of recognition to outsiders as Mario, but to PlayStation fans, those franchises are what keeps them coming back to Sony's consoles/handhelds. The fact that most of the titles above are making their way to the Vita is enough to get any big PlayStation fan in a tizzy.
I don't particularly think "killer app" is a helpful phrase, and I find it very subjective. What is a killer app for you, may not be a killer app for me or anyone else.
For example, I see in this thread many people snagged a Vita for Hot Shots. Some for Lumines. Some for wipEout 2048. For them, those games were the "killer app." For me, I snagged a Vita for Uncharted: GA. I'm also getting wipEout 2048 and Rayman, although I'm not a diehard wipEout fan.
Claiming that the Vita doesn't have a "killer app," is really only true in the sense that there isn't a singular title that will drive people to the store in droves. I imagine, in Japan, Monster Hunter will be that game. Here in the US, I can't say, but I'd imagine a CoD would entice a diehard fan who wants to take that experience on the go, and thanks to the dual analog sticks of the Vita, I imagine a Vita CoD would feel very good. Also, looking at the graphical prowess of the Vita, I can also imagine that would be another selling point to a fan of the series. I'm not a CoD fan, so that certainly wouldn't have been a driving force behind me picking up a Vita.
I could see Assassin's Creed being a draw for me, personally, but so would a Vita Ratchet and Clank, Sly Cooper, Jak and Daxter, or inFAMOUS.
Some of us PlayStation fans look at Sony's first and second party titles with as much excitement and fervor as Nintendo fans do at Mario, Zelda, and Metroid.
Just thinking about a Vita Ratchet and Clank, with the quality of A Crack in Time, or an HD remaster of the original 4 titles gets my heart all aflutter. A Vita 3D Dot Game Heroes would be heaven as well.
There is a market for Vita versions of legacy PlayStation franchises, even if they don't have the mass appeal of Nintendo's franchises.
Diversity has always been Sony's thing with the PlayStation consoles. There's certainly overlap among fanbases, but I always thought they catered to the "not everybody likes the same games" better than any other publisher in the industry. So much variety to appeal to equally diverse gamers and gamer tastes. The Vita launch is showcasing that with it's lineup, so I can't imagine how it's going to be when the system hits in stride in 6-12 months.
(sorry for the long post!)