The largest new feature is that it is now possible to include other files into the RML by using the "include" tag. This is useful for making markup files more modular. Also added some more options for existing things (e.g. built-in detail options).
Since libRocket requires unicode mode this doesn't change anything but it fixes an error when a mod uses TrueType fonts without unicode mode.
This new system allows to create a user interface that goes beyond the existing UI system.
Goober5000