I had a brief Twitter conversation with Anand Shimpi of Anandtech about this, and he was as perplexed as I was. Nobody could explain the technical basis for this vast difference in idle power management on the same hardware. None of the PC vendors he spoke to could justify it, or produce a Windows box that managed similar battery life to OS X. And that battery life gap is worse today - even when using Microsoft's own hardware, designed in Microsoft's labs, running Microsoft's latest operating system released this week. Microsoft can no longer hand wave this vast difference away based on vague references to "poorly optimized third party drivers".
The new Surface Pro 2 gets 6.6 hours of web browsing battery life. The MacBook Air 11", which has more or less the same hardware and battery, gets more than 11 hours.
I have a Surface RT - the first generation - and as such, I know why. Windows 8 might have Metro running on top of it hiding a lot of it, but Windows 8.x carries just as much baggage, cruft, and outdated shit with it as previous versions of Windows have. Windows 8/8.1 - and Metro in particular - simply suck. Slow, clunky, jarring, cumbersome, battery-sucking, restricted, and limited, with a crappy selection of rush-job, rarely updated applications. You know how resizing windows on Windows 7 or OS X is all nice and fluid? Why, then, is it a slow and jittery operation that brings Windows 8 Metro to its knees?
It's simple: just like battery life, it's a symptom of Microsoft's Windows team not having the balls to truly go for a clean break, as the Windows Phone team have done. And lo and behold, Windows Phone - even WP8, which runs on the same NT kernel - has none of the slowness and crappiness issues that continue to plague Windows 8 Metro (although WP has its own set of issues unrelated to these).
If you want a smooth, modern laptop today - get a MacBook. If you want a smooth and modern tablet, get the Nexus 7 or an iPad. Microsoft still has nothing to show for itself in these areas.