http://www.forbes.com/sites/ianaltman/2015/04/28/why-google-glass-failed-and-why-apple-watch-could-too/2/:
Google Glass failed to help consumers understand why they needed such a device.
Google Glass failed not because they didn’t educate consumers but because the product wasn’t ready for prime time. Even Google has said this. (Expect v2 to be ready with previous Nest CEO Tony Fadell in charge.)
With a recent move to larger and larger screens for users, it might be a risky move for Apple to shift its focus to a user interface the size of a watch face.
The screen size of the Apple Watch won’t be the reason it succeeds or fails. This device is in a completely different category than the iPhone. Even though it has some similar uses, the convenience of having it on your wrist will drastically change how we use the Apple Watch vs the iPhone. Even Apple Watch games will be complete different.