What's the name of this windows feature?

Sounds like UAC Virtualization.

Any writes to a system folder (or Registry) as a standard user were redirected to a location in the AppData folder so those apps that incorrectly assumed that all users run with full admin privileges would continue to work when migrating from Windows XP to Vista.

You are probably referring to the sandboxing done on processes with "low/medium integrity" flag. From Wikipedia:

Windows Vista and later editions include a "low" mode process running, known as "User Account Control" (UAC), which only allows writing in a specific directory and registry keys

Again, from another Wikipedia page:

Windows Vista defines four integrity levels: Low (SID: S-1-16-4096), Medium (SID: S-1-16-8192), High (SID: S-1-16-12288), and System (SID: S-1-16-16384).1 By default, processes started by a regular user gain a Medium IL and elevated processes have High IL.2 By introducing integrity levels, MIC allows classes of applications to be isolated, enabling scenarios like sandboxing potentially-vulnerable applications (such as Internet-facing applications). Processes with Low IL are called low-integrity processes, which have less access than processes with higher ILs where the Access control enforcement is in Windows.