Microsoft's signature Windows 10 Mobile feature called Continuum will soon be available to buyers across the world and while this is one of the greatest innovations of the Redmond-based tech giant, having it on your phone won't be possible unless you buy a new Lumia model.
Windows 10 Mobile, the OS that supports Continuum, will be released to all Windows Phone devices currently on the market, no matter their specs, but this particular feature needs special hardware to work. So what was only a rumor until now proves to be painful reality, as Windows Phone device owners won't be able to use Continuum no matter what.
As WindowsCentral notes, Microsoft confirmed that in order to run Continuum, you phone needs to be powered by at least a Qualcomm Snapdragon 808 MSM8992 or MSM8994 chipset, which is only available on the new Lumia models. So there you go, if you really want to have Continuum on your phone, you have to purchase either the Lumia 950 or Lumia 950 XL.
This means that not even the previous Windows Phone flagships, such as the Lumia 930 or the Lumia 1520, will get Continuum, despite the fact that they were supposed to get every single feature available in Windows 10 Mobile.
Lumia 930 and Lumia 1520 both come with a Qualcomm MSM8974 Snapdragon 800, so they do not qualify for the official Continuum requirements.
As for other features that might be missing from your Windows Phone flagship when upgrading to Windows 10 Mobile, you're also very unlikely to get Windows Hello, which helps secure your device using biometric authentication. This particular feature requires an iris scanner or an Intel-based optical device and since the existing Lumia flagships running Windows Phone don't have any, they won't be able to use it.
Other than that, everything should be there and working fine, but devices with only 512 MB won't get the full Windows 10 Mobile package because of their limited hardware capabilities.