It depends on what you mean by killer... For me, the killer feature would be not allowing used games, because it would kill my desire to upgrade. I buy 9 out of 10 games new, but I don't want to lose the option. I'm also not thrilled about the rumor of requiring the Xbox to always be online. Most people aren't fooled by the marketing speak on the subject. Always online is a vehicle for DRM.
But, in the spirit of your request, I'm simply looking for more parity with the PC experience -- performance and graphics.
On another topic, Microsoft needs to put out an update to the Channel 9 app that allows you to add comments.