There's a lot of discussion here about what were the killer apps for other consoles, but what is lost is that hardcore games don't dictate the market, its the casual titles, specifically sports & driving games.
If you follow the history of video game consoles all the way back to the 80s, at least in the US, the trend is that the tipping point was hit by the quality of sports titles, and as the Genesis and Super NES went head-to-head, the EA Sports franchises in particular. The most successful consoles have always been the ones that got the staples like Madden, NCAA football, etc ... first. These games, taken individually, don't appear to be system movers in and of themselves, but the games as a collection appeal to a broad market. For example, Madden targets NFL football fans, while the NCAA series targets college football fans. Taken individually, the two games aren't on par with other smash hits in the casual/sports markets. Taken together, however, the two titles move a lot of systems.
The Genesis fared well against the SNES specifically thanks to the overwhelming effort EA threw behind it with the hundreds of sports exclusives it had. The PlayStation brands absolutely crushed everything thanks to EA completely abandoning SEGA, when SEGA foolishly tried to get into the sports act themselves, and Nintendo after the last Madden 64.
As goes EA, so goes the console war. This has repeated itself the last three console cycles.
Nintendo now has EA back in the fold, which is a major coup for them, especially given the limitations in the horsepower of the Wii vs the other two next gen consoles. Nintendo's strategy to bundle Wii Sports was brilliant, and I believe calculated, to tip the sports world back to their favor, which is key.
As for what "killer app" Nintendo needs to make this whole thing a slam dunk: I think it'll be a Madden/NCAA style football game, or some sports game, that leverages the DS as the controller, similar to what Pokemon Battle Revolution does with Diamond and Pearl. I'm thinking of a scenario where you pop in your Madden football disc, start up your DS to connect to the Wii via Download Play, and you have the playbook pushed to the DS, so all of your play selections are 100% discreet. Additionally, the plays can be modified on the fly, such as drawing new routes for the receivers to run, simply by drawing a path on the touch screen with the stylus.
What EA needs is a revolutionary new Madden that finally reminds everyone who bought the last gazillion copies of it that this year, they HAVE to upgrade. The DS/Wii is the only platform where that could happen.
If a football sim had DS interoperability where 100% of the control was moved to a private handheld screen that you could draw/diagram on, the sales numbers, IMHO, would just be absolutely obscene.
