In Windows 8, All Apps is accessed via swiping up at the start menu (or click the small 'down' button with mouse/keyboard, which I have to say it's really not convenient for keyboard/mouse environment).
However in Windows 10, the All Apps need to be access via the "All Apps" button which is located at the bottom left of the screen, and it shows you a pretty small sidebar that contains "All apps" in.  And it's vertical, when you operate in one hand your thumb really need to scroll a lot, so it's not really awesome at all.  So two hand?  Since Surface Pro 3 is already pretty big, most of the time you are using both hand anyway.  But you still have to move your right hand (or left) to the left side and scroll through it, so why not just follow windows 8 that it's visible as whole screen since you're going to dual-hand anyway?  And the fullscreen "All-apps" in Windows 8 display more items in one screen, so you scroll lesser with that.

I was really happy at first when I realize every app/window is treated as a same thing.  Microsoft call it "Universal app" design I believe.  You no longer have to switch to Desktop and search the running applications at the taskbar there, it's all accessible via the app switch screen (swipe from left).  Finally no more awkward "Desktop" option in "App switch" screen!

But when you open up the virtual keyboard and the app windows resized to half of the screen to make space for the keyboard, and it doesn't back to fullscreen when you close the keyboard!  I tried "Maximize/Restore/Minimize" and they do nothing, the size remained half. And sometimes bugs happens that already stated by mrl, the keyboard appear but the app doesn't resize, and blocked the input box, which is really serious.  Don't get me wrong, this is actually a very good idea, but it needs a lot of improvement to make it good.  And the resize looks horrible as it flickers and distorted for like a second before it confirm the size.  It really feels like someone wrote an unofficial cheap script to make the automatic resize.

App-switch, I used to think Windows 8 app switch is quite weird especially when you swipe from left abit and go back, it shows all the apps in the blacky side.  In Windows 10, you get a full-screen app-switch when you swipe from left, it's better than the blacky side.  But why I can't quick switch between app?  If I swipe from left twice all it does it just close the opened app-switch screen, and you're back to the same app.  And the full-screen app-switch is positioned in center so you have to move your 2nd hand to switch apps.

Microsoft Edge, they promised to deliver another update in October that should bring it to another level that gives what it supposed to gives.  But at current state it doesn't support gesture operation (you can swipe to go back/forward in Metro IE you know?), no fullscreen (address bar always visible and there's no option), and it actually a striped-down IE that performs slightly better than IE but provide less features (no Save as Target is really...gah).

And another good point stated by mrl, the start screen now scroll in vertical.  This is actually not a really serious problem and I can get used to it, but I think scroll in horizontal suits Surface device (or any widescreen device) better. And, unless you're using Windows Phone, in most touchscreen devices (Anrdoid, iOS, etc) you actually scroll in vertical to find the apps, right?

Control center is pretty neat.  I actually like this feature, notification is probably one of the most frequent used functionality in every device nowadays.  I don't really have any complain except this: Why there's no slider for brightness? You can only change brightness by tapping "Bightness" button and what it does is just switching brightness value (25%, 50%, 75%, 100%).  The brightness slider is available in Display in Option page but, I have to navigate 3-4 pages to get there just for this common feature?

OneNote in Windows 10.  Oh god why they decided to do this?  OneNote desktop cannot go fullscreen mode now, the pen purple button now only open up Metro OneNote no matter how you change the option in Surface app, and the new Metro OneNote removed some important features as well (no pen size option?!)

So in Windows 8, if you want to choose a file/browse drives in metro apps, it shows a full-screen black file picker, and it's very inconvenient to use.  In Windows 10, they just throw in the usual Windows Explorer, but it's in window (not fullscreen), and less touch friendly.  Ok, I can let this go, but I'll miss the full-screen file picker for sure.

Graphic/app display glitch happens more frequently in tablet mode.  Ever seen the Continuum window (the one ask you whether you want to switch mode or not) got blocked by taskbar?  It happened more than once.  Password picture wait for 1 second then flicker, before it flicker all input is blocked.

Of course I'm more bias to Windows 8 because I'm get used to Windows 8, but I need to point out the fact that touchscreen in Windows 8 is more enjoyable.  I just can't understand why Microsoft want to change something that is actually not a problem?  Desktop mode was bad I agree, so I love the changes to desktop mode in Windows 10.  But toushcreen mode in Windows 10 is definitely much less polished and less useful compared to Windows 8.  It's like Microsoft develop Windows 8 with a touchscreen device all the time, and now they develop Windows 10 with a desktop that have a touchscreen monitor.

Action center!  Notification, shortcut to several options, pretty useful.

There is option to hide the pen cursor so that 'lagging' cursor (it's actually really lagging anyway) won't annoying you anymore.

Apparently Windows update can get files from another Windows 10 that is within the same local network, means you can update faster!  Finally no more "you-cried-when-you-have-7-win8-devices-need-to-update-to-win8.1-and-each-of-them-need-to-download-3.8GB-from-windows-store" situation.  BUT, apparently by default it is set to "share to local and Internet" so I think if you have that checked, it uploads windows update files to another unknown user in Internet, just like you're seeding torrent.

I never thought the upgrade can be so painless.  Kudos to Microsoft.
...And I never thought restore can be so painless as well!

Virtual desktop.  I can use Sysinternal Tools virtual desktop but built-in is better.

You can finally see the estimated hour left when you're using battery when you check battery.  Although I wonder why it wasn't available in Windows 8?

CTRL+C and CTRL+V now FINALLY works in command prompt in Windows 10.

Of course it is the best when metro apps is resizable in desktop mode.

There's a battery saver mode!

Desktop mode is smooth, and better.

Option app looks much better. MUCH much better.