So...this is the official beta? Is the decision to go down this route final? If so, my first and biggest concern is the compatibility. Do noteskins and themes of both 3.9 and 4.0a have to be remade? I'm no coder, so I don't know how Lua->XML transition affects the game in general. I can only guess it's going to be very different from previous versions.
Personally I don't like where this is going, but I'll wait and see.