The main highlights are the support for 64-bit applications, and the new graphics based on the Tango standard. But it’s hard to say what the best change is. There’s just so many!

QEMU is a generic and open source machine emulator and virtualizer. When used as a machine emulator, QEMU can run OSes and programs made for one machine architecture on a different machine architecture. This is different than VMware, VirtualBox, or Hyper-V where you’re running different operating systems on the same architecture.