personally, i think a lot of stuff - everything they announced with multitasking, the way the all apps page works, the way wallpapers work, lockscreen functionality, ect - shoudl have been that way since day one.
i'm very intrigued by 7"/8" tablet support.
i still plan on avoiding don't-call-it-metro apps like the plague, but i saw a couple live tiles, like the weather, that actually seem potentially useful to me.
using the new start screen as the start menu's replacement seems a bit easier now, from a desktop user's perspective.
unified search - about damn time
skydrive integration. ugh. why.
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that said, the two features grabbing the most amount of PR are the two i am the most unsure of - the new/old 'start button' and boot to desktop.
thing is, though they've inherently changed how you interact with the desktop, the backend hasn't changed. In 8.0 everything was very clear cut. Metro start screen, metro apps, ect. The desktop was an app, a shelled environment for running your legacy software. While i didn't/don't agree with this, it was VERY clear cut. Metro is/was microsoft's vision for the future, the desktop is on its way out.
now, in 8.1, the lines are very blurred. what type of software is microsoft pushing again? the desktop is now "ok" to use again, vs the "we'd really rather you stayed out of it" feel of 8.0. I mean seriously, use some metro apps, and go back to the desktop. it feels like you're living in an emulator, and that's not too far from the truth. but now, the mechanics behind it haven't changed, but they're artificially elevating the desktop to an "importance level" of much higher in the OS.
Its really hard to see where microsoft is actually heading now. desktop? metro? its hard to say, and i don't like that.
TLDR; very interesting. actually an OS i won't mined using, a lot better than W8.0, but seems as if it can't make up its mind.