What constituted a Killer app 3-4 years ago, when the iPhone hit the market, probably isn't one any more. At the time, decent web browsing on a mobile truly was a killer app because none of the alternatives (RIM, Palm, etc) had decent web browsers. So it was essentially exclusive - if you wanted a great web browser on a phone, the iPhone was your choice. Now it's basically a commodity feature, since all of the major mobile platforms have a workable web browser.

"Exclusive" is maybe too strong a word, since it implies that no other platform will ever have that Killer Feature / App, e.g. Halo. But if only one platform has it when you need to buy, it's for all intents and purposes an exclusive.