Okay Microsoft, let's have a chat. I've been in love with the Microsoft Vision since you dubbed Windows Phone 'Windows Mobile'. The idea of all my life essential gadgetry playing nicely together was exciting, and now with your Windows 10 Pitch I'm even more excited. So let's talk about how this can go from awesome, to Framazing!
First thing's first. Let's talk about the Desktop + Cell Unification. Way back in the Windows Mobile Days, there was this beautiful little app know as 'My Mobiler'. With it you could fully use your mobile phone through a window on your Desktop. Since then, this has kind of become 'a thing', (And you've even released a 'Project my Screen' app that you've failed to update since launching it... ;?) and if you're truly serious about this platform being 'One System across multiple devices', let's see that here.
There is no reason (Considering that most of what I'm suggesting exists in Apps on Other Platforms.) that the phone and the desktop cannot FULLY play together if it's designed that way from the get go. Especially now that you log into the Windows 10 Desktop with your Microsoft Account. If you log into a Desktop, and a Phone, with the same Microsoft Account, from that point on (Assuming you select the 'Sync all the Things' options, which should be presented as an option when you first log in) you should be able to use either device for any phone related functions and have access to: Call history / Visual Voicemail, SMS / MMS History, Buying / Downloading Apps, Setting Alarms / Reminders, Etc. just as easily from either device.
Now, when the devices connect, you've got the potential for some serious awesome now. Bandwidth across all usable 'Connection' methods on relatively modern devices (Wifi, Bluetooth, USB), is plenty sufficient to fully utilize the hardware in either device from the connected device (And when it's not, a message indicating to the user that they need to utilize an alternate connection can be generated). So once the devices are connected, the PC should now have access to all the hardware on the phone (Cell Antenna, Wifi / Bluetooth Antenna, NFC Reader, Camera, Touch Screen, Speakers / Screen, Gyroscope, Etc,) and the Cell should be able to take advantage of whatever hardware the PC has connected (Especially the Second Screen, Keyboard, Connected Cameras, Speakers, Etc.), even if some of these connections require specialized app interfaces for them (Where others can just show up as 'Connected Devices' in the Windows Device Manager and thus become available to other applications, Both Ways).
Specifically, with a suitable connection between devices, a user should be able to make calls and send text messages from their desktop, manage any of the settings on their device, use any application they have installed on their device (Even if some of them must run through emulation, or screen projection when core device functionality is necessary), receive all notifications on the desktop, and as the device should be recognizing the hardware from the PC, they should also be able to utilize the keyboard, screen, and speakers for the device (Even if it's only through screen projection in necessary cases).
THIS would make Windows 10 the truly unified OS they've been advertising. And while some of this functionality may need a little baking time to get right, having all of this on the roadmap in a manner that any end user can access without needing to look for Unlocked Device Apps would Truly set Windows 10 in a position above the rest. (And even though I'm only addressing Phone and PC Synchronization here, this One Platform vision should apply to the Xbox One and any other Microsoft Devices... and maybe I'll explore that in a future post).