Hi Anand,
I am not quite sure in how far you question is related to the Windows Research Kernel, but here is Wikipedia's definition:
"A killer application (commonly shortened to killer app), in the jargon of computer programmers and video gamers, has been used to refer to any computer program that is so necessary or desirable that it proves the core value of some larger technology, such as computer hardware like a gaming console, operating system or other software. A killer app can substantially increase sales of the platform on which it runs."
Best regards,
Alex
http://www.dcl.hpi.uni-potsdam.de/research/WRK