The first major change in this release is the input system. The way that the OS-specific layer communicates with the core about input devices has changed substantially, allowing a lot of the tricky functionality to be handled automatically by the core code. For the Windows build specifically, this marks a shift in behavior: on Windows XP and later, both keyboard and mouse now use the raw input APIs, which allow support for multiple keyboards and mice.
A second big change is an experimental one. A new built-in user interface is provided for selecting a game. This interface is intended to be minimal and will not serve to replace a true frontend. However, it does provide a better first experience for those unfamiliar with MAME and is useful for launching a game when you can't remember the exact driver name.
In addition to these changes, we have discrete sound for Head On and clones thanks to couriersud; an updated Monza GP driver from Phil Stroffolino; and some nice fixes to the toaplan2 driver from Alex Jackson.
As always with big changes like this, make sure you test things out and report any strangeness over at MAME Testers. Have fun!