Totally new config system, which means re-configuring ....but for the last time - config files will auto-update from now on
Sound / music code re-written and improved with new features,
Movie support (intro/exit, previews, screensaver),
Fully configurable input system to allow multiple keypresses/combinations, mouse & PC joystick device support, and some new navigation features.
Doesn't look a lot there, but 50% of whats now in MAMEWAH is now totally new or re-written...
Due to the massive differences to previous versions, I will recommend you install to a fresh directory. While you will have to re-setup your emulators & general settings, as I said above it should be for the last time. Layouts will still be compatible, but you'll have to rename some files.