How do I reverse selected lines order in Vim?

To reverse all the lines in a file,

:g/^/m0

For an explanation see

:help 12.4

which also shows how to reverse just a range of lines.

Select the desired lines, hit !, and in the resulting prompt pipe the lines through tac a la :'<,'>!tac. See man tac for more details.

On Mac OS X, tac does not exist, but you can use tail -r to the same effect:

:%!tail -r

This also works nicely for visual mode:

:'<,'>!tail -r

Excerpt from tail(1)'s manpage:

The -r option causes the input to be displayed in reverse order, by line. Additionally, this option changes the meaning of the -b, -c and -n options. When the -r option is specified, these options specify the number of bytes, lines or 512-byte blocks to display, instead of the bytes, lines or blocks from the beginning or end of the input from which to begin the display. The default for the -r option is to display all of the input.