From the feature:
Let’s start with the biggest difference. Either this is just a bug or it is a feature. I really hope it is a feature as it would be very great to have it on such a big screen. The Lumia 1520 is a mix of smartphone and tablet. In portrait mode the phone works just as every other Windows Phone. However, now more of the OS can be used in landscape mode, which is a difference to Windows Phone 8.1. Anyway, that’s not everything as one app scales like on tablets when used in landscape. This app is the new file explorer and it looks like this:
Obviously this looks a little buggy. The left element (in fact it’s the hamburger menu just without a button – it’s always displayed because of the size of the screen) is way too big, but I would really love to see this come to more apps in an improved form...
...The landscape mode is not that good in every app, though. The maps app for example looks very crappy in landscape as the elements are really badly placed. However, it’s a preview and everything might change. So far it is a huge step forward compared to Windows Phone since at least there now is landscape functionality and I really hope the 1520 will benefit from it eventually.
I’ve already mentioned the hamburgers above. While an always displayed menu makes sense in landscape, here universal apps could really shine if properly coded. In portrait mode the hamburger menu is simply crap (sorry for the word but it’s the only one that fits). The best example in the new music preview app. It’s gorgeous but the button on the top left corner is only disturbing. Pivots would do much better (basically like everywhere). The same for the new mail app. It’s gorgeous as well and very functional, but having the elements up top makes it almost unusable on the 1520.
Another thing I noticed is that the option menu on the bottom of the screen can no longer be opened by swipes over the edge of the screen. Especially in the new office apps this would make sense since it speeds the workflow: Instead of having to tap the three buttons one could simply swipe over the edge just as it works in Windows Phone currently. I hope that this is simply not coded yet but planned....
You can read the whole piece here.
Lucas also comments on the higher than expected battery drain, but we saw 'Preview for Developer' releases last year go through the same battery optimisation cycle and the final version was more efficient than the previous production release, so I'm not too worried here.
My own biggest observation of the Insiders Preview is that the interface is so slow, but one has to imagine that there's an awful lot of debugging code in the OS at this time and that things will speed up once this is removed.
If anyone's interested, I currently have the Windows 10 Insiders Preview flashed onto my Lumia 1020 and 630 - and am contemplating putting it onto the Snapdragon 800-powered Lumia 930 after the next Insiders release is up. Comments welcome on this risky plan!