After upgrading to 8.1 on my retina mac, I haven't found it much better than 8. Changing the DPI settings still breaks stuff until you logout, plenty of icons are still blurry, IE's support is buggy, and third-party app support is still scarce (e.g. Chrome is a no-go).
At what resolution does hi-dpi start?
anything not 1:1 basically. once you start scaling up resources / fonts / reporting pseudo-pixel positions you start running into problems.
Whatever you want to call it, I suppose. I don't think the Windows 8+ features are resolution-dependent. There's a new slider in the display control panel that lets you set the scaling factor. Windows 8.1's main change was adding a checkbox that lets you set a different scaling factor for each screen, and you can choose a custom scale, not just 1x/1.5x/2x.