A single feature so valuable that customers will tolerate weaknesses in the rest of the product to get it, and often switch from incumbents who do everything else better.
When Apple announced the iPhone in 2007 it had no third-party apps, no copy-paste, no MMS, and a slower network than competitors. None of it mattered because pinch-to-zoom multi-touch was unlike anything the industry had shipped. Within five years Nokia, BlackBerry, and Palm had collapsed in smartphones, and Apple's smartphone unit alone was worth more than most countries' largest companies.