A killer app for consumers probably wouldn't exist for most things, unless you could integrate streaming or delivery services.
But I think the real killer app will help correlate consumer behavior, advertising and the supply chain. You could have a refrigerator that keeps track of everything you buy and where you bought it. Scaled up to tens of thousand or millions of consumers, you might have very useful data for stores to use to predict what to stock and how much, and how to target advertisements. I'm reminded of the story of Wal-Mart predicting one of their customer's pregnancies based on their purchasing habits, and sent coupons for related products.
I also think integration with social media is going to be a huge part of the IoT taking off but I'm not entirely sure how, yet, other than vague ideas about appliances having their own AI driven social media accounts and posting to your feed. That could both be useful, and an incredibly sinister way of getting consumers to empathize emotionally with a brand, even if only subconsciously.