Here's a Rolex[0] (Sea-Dweller 4000). Here's a Moto 360[1].
And now instead of holding eWatches to the same standard as mechanicals, it has turned into another thickness race.
Here's Piaget Altiplano for example: http://blog.luxurybazaar.com/wp-content/uploads/2014/02/Piag...
or
Blancpain Billeret https://s-media-cache-ak0.pinimg.com/236x/62/39/fa/6239fae47...
Mechanical watches under 7mm are already considered thin; more usually they are 9-11mm. Quartz are easily thinner.
I think smartwatches currently around look thicker because of case and lugs design. For example, the Sea-Dweller is 18mm, but the side of the case is not flat all the way like the 360, the case back is actually inset, so it’s less noticeable.
IMHO it has one killer feature the Moto 360 doesn't - e-ink (which brings with it always-on display, bright-light readability and power savings benefits). For that reason alone I'd be way more likely to pick up one of these than a 360. I also like the design a lot more (huge bezel and all), but that's even more subjective.
I'm not going to get the Pebble Round, but I'd be far more willing to strap one to my wrist than I would the Moto 360 monstrosity.