If half the tech Apple is allegedly investigating ends up in the final product, I think it will be a disruptive device-- and definitely one driven by Apple's expertise with software, ecosystem integration, and device-appropriate interfaces.
I strongly suspect the iWatch (or whatever) will have the typical fitness bracelet stuff-- pedometer, calorie counter, something to measure sleep quality, etc. But if the company goes beyond that (measuring oxygen in the blood, measuring blood pressure and skin temperature, measuring glucose levels, even predicting hear attacks if last weekend's SF Chronicle report is to be believed), we're into new territory. I've heard a few high-ranking Cisco execs argue (in the context of their Internet of Everything campaign) that wearable devices embedded with medical sensors will be one major component in the path to better disease detection, more physically active citizens, and ultimately more and more people living a century or more. I don't know how quickly fantastic increases in longevity will come on board, but I do believe wearable devices could meaningful benefit the user's health. Based on reports, Apple's iWatch could be the first one to seriously test this belief.