Windows 8.1 entirely removes the check box for XP style scaling and always enforces the default behavior, i.e. DPI virtualization is never used at 120 DPI but is always used at higher pixel densities, for those programs that don’t declare themselves DPI-aware. If some applications suddenly appear blurry, you must manually opt them out of DPI virtualization.
Windows 8.1 allows multiple monitors to use individual DPI settings. However, this feature also forces DPI virtualization on legacy applications that are dragged between monitors with different settings. To avoid this you can disable per-monitor DPI scaling globally, using the new option “Let me choose one scaling level for all my displays.”
Lastly, Windows 8.1 adds a dedicated radio button for the existing 200% setting, and a new API to let developers selectively disable DPI virtualization. See my review for more information from a user perspective, and my earlier preview coverage with some links for developers.