Dollar-paren expansion beats backticks for being nestable.

E.g.: I like dumping temporary files with timestamps:

some-command-to-generate-log > /tmp/log-$( date +%Y%m%d-%H%M%S )

Now, if I want to wrap that in a larger loop -- say, iterating over a number of files or parameters:

command $( expansion one $( expansion 2 ))

Works

With backticks you'd have to do escapes:

command `expansion 1 \`expansion 2\` `

... which gets tedious.