The two new features for Linux 3.8 with EXT4 are Inline Data and SEEK_HOLE/SEEK_DATA support. Ted Ts'o mentions that the inline data feature allows small files or directories to be stored within the in-inode extended attribute area. This inline data assumes that the file-system uses inodes that are 256 bytes or larger.
The SEEK_HOLE/SEEK_DATA support is enabled by the extent status tree patch-set and is infrastructure that will be used to further optimize the EXT4 file-system moving forward.
The other Linux 3.8 changes for this common file-system just comes down to clean-ups and bug-fixes. The merge can be found already in Torvalds' tree.