I think Windows 10 Mobile is far from finished, but I see a lot of potential for an OS that can be both the most complete and most beautiful of any OS. Already, when it comes to basic features, we have a lot in Windows 10 Mobile. There's obviously a bit missing but it'll all come in time.
And I like the new design language. I like the subtle mixture of circles and squares, the detailing with shading. The Light theme needs work, as does the Lock Screen and in my eyes the Action Center and Settings page has a long ways to go too, but it's all going in the same direction, and let's face it that's not something we've been able to say often.
Microsoft is seriously trying to bring their platform together and if we don't show them that we know exactly what we want, then Microsoft will only put half the effort necessary into Windows 10 Mobile and it'll flop. Let's put three times as much effort as necessary into making this OS amazing.
The Light theme needs work with shading. Ironically, the Dark theme looks more colorful, and the Light theme is also inconsistent. Plus, Microsoft apps at the very least need to support both Dark and Light themes completely.
The Lock Screen is just bland. It needs to have more customization. In Settings, we should be able to add features, such as Lock Screen music controls (I want these), Weather, and a Quick Action or two to launch our favorite app quickly.
The Action Center faces the same issue as the Lock Screen. It still doesn't look like Windows 10 Mobile. It looks like Windows Phone 8.1 except with a few more boxes and smaller font. The Quick Actions look overly blocky in my opinion. Make them a little wider, just by a few pixels. They don't need to be perfect squares, and they'll look way less awkward without them. Give us the option for aero transparency. We should be able to choose between opaque them (solid theme color), aero theme (blurred transparency with theme color) and aero accent (blurred transparency with accent color). Let us customize the Quick Actions, I want to move them around to where they're best for me! Give us more quick launch buttons for apps that we can choose to have there or not. Move the All Settings button right below, to the center, with Expand/Collapse to the right and Clear All to the left. Improve the gestures. Right now I can expand with a swipe but I can't collapse with another swipe up. And give us back that little bit of feedback we had when we held down the bar at the bottom of the Action Center. Use the space in the Quick Actions better. Widen them a tad, make the icons a bit bigger, just avoid that awkward look we have now. And make the notifications more useful. I love how they look now. I love how I can interact with an alarm that just went off from the Action Center. Let me do the same for every other Microsoft app.
The Settings page is inconsistent and incomplete. The Extras page needs to match the rest of the Settings. I don't understand why it's different and it really bugs me. Also, coagulate the Settings for all the default apps in the Settings page! Let me access the Settings for Music in the Settings app. Make the All Settings button in the Action Center truly be for ALL settings. I think it would be AMAZING if I could go into the Settings app, open the page marked "App Settings" and customize every default app on the phone, be it OneDrive, Outlook Mail, Microsoft Edge or whatever.
There's plenty of UI changes I'd make throughout the OS as well but they're too numerous to put on here.