EXT4 err: couldn't mount because of unsupported optional features

Backward & Forwards compatibility is great! Until it bites you.

The extN branch of filesystem is the pretty much the standard on your average GNU/Linux installation. Migrations tend to work well if N is increasing i.e. from ext2->ext3->ext4. However, things are not so rosy if you want to mount your ext4 disk on your old PC.
Ext4 which made it into mainline in 2.6.27 introduces new features such as extents that are incompatible and unknown by older kernels. Attempting to mount an incompatible extN fs on your old kernel will fail and err out with logs similar to:

EXT3-fs: sda1: couldn't mount because of unsupported optional features (240).

EXT2-fs: sda1: couldn't mount because of unsupported optional features (240).

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(8,1)

The kernel here attempts to probe and mount the using the most recent extN. If that fails it tries the next ext filesystem and so on. In this case, this is a 2.6.16 kernel attempting to mount an ext4 fs with new features.

# dumpe2fs /tmp/img.ext4
.....
Filesystem volume name: /
Last mounted on: /mnt/tmp
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags: signed_directory_hash

 The solution in this case is to use a newer kernel that understands ext4 or recompile your kernel to support ext4.