From the announcement: "This release fixes some bugs discovered in the 6.4.0 source code and a few new features (over 560 updates to the source code with respect to 6.4.0 have been added). As a stable release 6.4 will enjoy long-term support.
GRASS 6.4 brings a number of exciting enhancements to the GIS. Our new wxPython graphical user interface (wxGUI) is debuted, Python is now a fully supported scripting language, and for the first time since its inception with a port from the VAX 11/780 in 1983, GRASS runs natively also on a non-UNIX based platform: MS-Windows."
Here's the overview of changes for 6.4.1, including new modules and major module changes.