As opera left its toddler years behind, it grew more restrictive and extravagant at the same time. Around 1700, a new style called opera seria began to dominate. It was, as the name implies, "serious opera," and was driven by two main forces: formulaic librettos and flamboyant singers.
The men who wrote opera seria texts — librettists such as Pietro Metastasio and Apostolo Zeno — were concerned with order, dignity and tragedy. They purged almost all of the comic elements opera had naturally absorbed and wrote stories that were serious in tone and clear in structure. Typically, conflicts between lovers or authority figures (or both) would be resolved not by some mythological god swooping in at the last minute, but by a benevolent, morally upstanding ruler.
Metastasio had a near monopoly on these tales. His 60-some libretti engendered almost 1000 operas, written by countless composers over several generations.
In opera seria, the number of characters is limited. No more hordes of people clog the stage when six or seven characters (three lead and three or four supporting) will do. Each receives an appropriate number of arias crafted in the strict da capo formula — an A section is followed by an emotionally contrasting B section, then A is repeated but with luxurious ornamentation, giving singers a chance to shine.
And shine they did. This was the age of the rock star castrati, the superpowered male singers who were castrated before puberty to preserve their high voices. The combination of such high tones produced from the chests of full-grown men made for virtuoso singing that has not, we're told, been matched since. (To hear the so-called "last castrato," though weakened by advanced age when the recoding was made, click here.)
The composers of opera seria — including Handel, Vivaldi, Alessandro Scarlatti, Nicolo Porpora, Johann Hasse, Giovanni Pergolesi and later Salieri — wrote flashy, complicated and grandstanding arias especially for the castrati, but also for the other voice types. The glittering arias followed one after the other like pearls on a string. It was a practice that would prove to be opera seria's undoing, when a composer named Christoph Willibald Gluck decided it was all just too much. But that operatic reform is the subject of another discussion.