This blog is updated daily.
A general description is here.
The way packages are installed has been overhauled to be much closer to the Unix version: for example Rcmd SHLIB is used to build DLLs (unless there is a src/Makefile.win). This means that various private variables and files that some packages used are no longer available, notably DPKG (the documented variable was and is R_PACKAGE_DIR) and MkRules (use etc/Makeconf).
Rcmd INSTALL now supports the same option names as the Unix-alike version: use 'Rcmd INSTALL --help' for details. In particular, --docs is replaced by --no-docs, --no-text, --no-html, --no-latex, --no-example and --no-chm.
Since iconv.dll has been renamed, packages wanting to link against it need to use -lRiconv (or, hetter, use the entry points in R_exts/Riconv.h).
Rcmd INSTALL now supports the same option names as the
Unix-alike version: use 'Rcmd INSTALL --help' for details.
In particular, --docs is replaced by --no-docs, --no-text,
--no-html, --no-latex, --no-example and --no-chm.
Package installation will authomatically skip CHM help if the
HTML Help compiler is not found (instead of trying and failing
as before).
iconv() is now done by a version of Yukihiro Nakadaira's win_iconv rather than by libiconv. This version is based on Windows' codepages and is not quite as comprehensive as libiconv: it is however much smaller and easier to maintain. The implementation here is 100% compatible: you can drop in libiconv's iconv.dll as a replacement if you need it, from http://www.stats.ox.ac.uk/pub/Rtools/Riconv.dll.
To avoid name conflicts with GTtk+, it has been renamed to Riconv.dll.