It's basically 8 with a much better thought out design. The modern (metro) apps opening in a window in desktop mode is huge. The thing that was huge for me was that they added a bluetooth on/off toggle now.
This. My favorite part of Windows 10, though, is the fact that they finally unified Settings and Control panel. That there were two different places to go for settings was the biggest annoyance when I started using Windows 8. Control Panel is still there (I'm not using the latest or final build, so I don't know if it's going away), but as far as I can tell most of the standard control panel options are now in Settings, as well.
Other than that, I really didn't have any issue getting used to 8 and 8.1, though Continuum seems like a no-brainer feature that should have shipped with 8. I kinda miss Charms - I sometimes push my cursor to the corner and edge of the screen before remembering they aren't there anymore - but I get that relatively few people either liked them or even knew they existed. The Notification center up for it, though I don't really need the notifications themselves (at least not right now).
Spartan/Edge is a pretty nice browser. Quick to start and render pages. I'm not going to use it over Waterfox, but I can see myself using it on a tablet. Microsoft has come a long way since the old days of Internet Explorer.
I've been using the Preview for 10 on desktop for a while, but I haven't tried it out on tablet yet... I'll probably wait to get a new tablet and upgrade to 10 when it's official. So far 10 feels just like a polished version of 8.1, which is fine by me since it's been my favorite version of Windows.
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