Indeed, true killer apps are tied to the hardware in some way. It's easy to forget this in hindsight because this is usually a matter of computing power. Take Visicalc, for example. It was the first killer app for personal computers, and it could probably be run on some calculator watches today, but it fit into a unique niche when it was released. Visicalc could have been done at the time on a different platform -- mainframes -- but it wasn't a good fit on that platform. Computing time on mainframes was too expensive to waste on something so trivial. At the same time, personal computers weren't powerful enough to perform the same tasks as mainframes. Combine the right application with the right platform, though, and you solve both problems and make a mint.
These days, especially in video games, it's too often the case that a killer app is only really tied to its platform by legal matters. There was nothing about Halo that tied it to Microsoft's platforms other than the fact that Microsoft owned all the rights, for example. I think Nintendo has been aware of this for a while. Super Mario 64, for example, was essentially designed in tandem with the N64 controller. This made the app-platform relationship stronger than it would have been if it were merely a matter of pushing enough polygons. It really wasn't until the GameCube that Nintendo tried to do it the other way, and the GameCube suffered something of an identity crisis because of it.