For me, it's all about notifications and the haptic feedback. Discreet notifications that I won't miss. This changes a lot of things.
Reminders and calendar. No more reminding yourself hours in advance for something. Say I need to call so and so client at 2pm. In the past, I have that in calendar. I review in the morning and periodically. I may even set a reminder 15 or 30 mins prior so I don't forget and hope i check my phone in time. With the watch, I can set that reminder at 2pm or 1:59 and feel confident I will get it..just in time. I like to call them "just in time" reminders.
Missed texts or phone calls. I think that's obvious. The trick will be which ones you want coming through the watch. I can still choose to ignore them or wait til later to deal with it (or handle it quickly with a short reply) but I'll know about it at least. And knowing is key.
Time and billing. I bill for time. Hope I can use watch for this.
Stocks. Perhaps a stock as reached a certain point you were targeting, you get haptic feedback about it.
Ebay. How about that auction you thought you might like. Deadline is approaching. Haptic feedback. I never bid til last second.
I can go on, but most things will have to do with the taps and haptic feedback that only the watch can do. It's what is unique about it compared to the iphone. It's what your iphone can't do. I haven't even really gotten into full 3rd party apps because that isn't something I really care about. Like the ebay example, I just want to know about it, maybe provide a button to tap to place bid (though you wonder about lag), that's it. I haven't gotten into the health stuff at all because I don't care yet. Or controlling things like apple tv because I think things like this will suck compared to simply pulling out your iphone. Siri would have to help out here a lot I think but I think Siri needs to be on board first as well as these type of apps.