The aforementioned article about the Apple Watch talks of "heavy application use" and suggests it might only last 2.5 hours. If you read, for example, http://www.trustedreviews.com/iphone-6-review-battery-life-a..., it suggests "heavy use" of the iPhone 6 (at 50% screen brightness) yields about 14 hours of battery life. That's a pretty significant difference.

While it's easy to say "it's more compelling to do stuff on a phone than it is to do it on a watch, so you'll naturally use the phone more", that rather makes me wonder why I would bother buying the watch, especially when you actually need the phone as well.

Time will tell how people use these devices and whether the battery really holds up to a reasonable person's usage. And, of course, it seems likely (though not certain) that later generations will have improved battery life.