The portrait on this coin is what I like to call the "Fat tyrant" portrait. Earlier coins of Nero show a much more slender and younger version. Although he did not fiddle or play the lyre while Rome burned, he did mange to spend the imperial coffers quite extravagantly. His excesses, including many building projects meant that Nero had to make the silver for coins stretch to provide ample coinage for the empire. In order to do this he debased the denarii. While they were once near 100% fineness, Nero reduced this considerably. He also reduced the weight of the denarii. This is one of the reasons why some early denarii are scarce to very rare. People hoarded or melted down the earlier denarii. This includes the coins of Nero himself. While coins of Nero after the debasement are relatively easy to find (but very popular), his earlier coins are quite difficult to find. If you are interested in Nero and see a pre-reform denarius recognize that you might not see another for a while.
This coin is also interesting because of the reverse. Notice it has both the legionary standards, and Aqilla-the legionary eagle. Vespasian was not the only one to use previous coin types. Legionary denarii were minted by Marcus Antonius. I do not know why Nero would want to recall these earlier days. However, the earlier legionary denarii were still circulating at the time of Nero's rule. they were made of poor silver and so they were exchanged many times over the years.
Nero. AD 54-68. AR Denarius Rome mint. Struck AD 68.