This blog is updated daily.
A general description is here.
Indexing with a vector index to [[]] has now been extended to all recursive types.
pairlists may now be assigned as elements of lists. (Lists could always be created with pairlist elements, but [[<- didn't support assigning them.)
The perl=TRUE versions of strsplit(), grep() and friends now work in a non-UTF-8 MBCS locale, by translating the inputs to UTF-8.
C99 support for MBCS is now required, and configure option --disable-mbcs has been withdrawn.
A suitable iconv() is now required: support for configure option --without-iconv has been withdrawn (it was deprecated in R 2.5.0).
Support for memory profiling is enabled in a standard build (this can be changed by editing config.h).
The help default has been changed to be HTML help rather than CHM help (which is currently unavailable). [Not yet the MSI installer.]
Binary packages are currently installed with all the types of help they contain [but that may change before release]out CHM help if they contain it.
The categories in the installers have been simplified. Now the files to support installation of source packages are always installed (they were now only 0.7Mb), and there are no longer categories 'chtml', 'html/help', 'latex' and 'Rd'.