If the child had been a girl, she would have become one of the very few monarchs-in-waiting who is female. As things stand now, the vast majority of the current monarchs are male. So are their heirs; so are the heirs to their heirs. Unless something dramatic and unforeseen happens, the world's 25 monarchies will continue to be overwhelmingly male for at least the next several decades.
Most monarchies forbid women outright from taking the throne; this includes all eight Arab monarchies as well as Cambodia, Lesotho and Brunei. Three countries allow female succession but only if there are no men available, which makes a queen very unlikely: Thailand, Bhutan and Tonga.
If there is a new queen in the world, she will almost certainly be European or possibly Japanese. Four European monarchies allow equal female succession, meaning that the crown passes to the oldest heir regardless of gender. The other three European monarchies are in the process of changing their laws to allow equal female succession; Japan is considering this change as well. With the birth of Middleton and William's son, it looks like most of the coming European monarchs will still be male. There are a few princesses in line, but most are still a ways off from the throne, some by half a century.
Right now, there are two queens in the world and 21 kings. It looks like that ratio is going to hold for the foreseeable decades.
Countries with likely future queens
Sweden: Next generation will be queen. King Gustaf is 67 years old. His daughter, 36-year-old Victoria, is the heir to the throne and can legally become queen.
Belgium: Next generation will be queen. King Philippe is 53, which means that unless he abdicates, he may hold the throne for some time. But his daughter, Princess Elisabeth, is also his heir. She's 11 years old, so likely not quite ready to become queen.
The Netherlands: Next generation will be queen. King Willem-Alexander is only 46, so also likely has many years left, but his oldest child is Catharina-Amalia, who is next in line for the throne. She's currently 9 years old.
Countries with possible future queens
Spain: Seven-year-old girl on path for throne. King Juan Carlos is 75 years old. Though his oldest child is female, Spanish law gives men privilege in royal succession, which is why Carlos's son Felipe is next in line for the throne. But Felipe's two children are both girls, which means that, unless he has a son or something unexpected happens, his elder daughter Infanta Leonor will become queen. That's many years from now -- she's only 7 -- which leaves plenty of time for Spain to change its law and allow equal female succession, as most Spanish want. That would formalize Infanta Leonor's ascent and ensure future gender-equal succession.
Norway: Nine-year-old girl on path for throne. Princess Ingrid Alexandra is only in line to become queen after her father and grandfather, probably a long time from now. If her father, 39-year-old Crown Prince Haakon, has a son, then Ingrid may lose her claim. That's not totally clear: A 1990 Norwegian law says women have equal royal succession rights, but that law has not been applied so far, though the king's eldest child is female.
Japan: Could allow female succession but probably won't. Japan's emperor is 79 and his sons are 47 and 53. For many years, the elder son had no children and the younger son had only daughters. Japanese law forbids female succession to the imperial throne, but the lack of a son would have meant the end of a centuries-old imperial line. For years, legislators and the public debated ending the men-only rule and allowing an empress, which would have been a major step for a country where women still have narrow public roles and are lightly represented in government. But in 2006, Prince Hisahito was born to the younger of the emperor's sons, providing a male heir. While Japan could still overturn the rule, there's little indication that it will.
Countries with nothing but kings ahead
United Kingdom: No queen for at least three generations. With the royal son's birth Monday, the next three generations of heirs are all men. Unless something significant changes in the royal line, the United Kingdom will not have another queen for at least these three generations, possibly more if Monday's newborn has a son as his first child. Given that Elizabeth II is 87 years old and still kicking, and that life expectancy is expected to continue rising in Europe, it will likely be a very long time until the next queen reigns over the United Kingdom.
Denmark: No queen for at least two generations. Queen Margrethe II, the only other queen in the world besides Elizabeth II, is 73. Her son and heir, future King Frederik, had a son in 2005.
The 12 countries that bar female royal succession outright: Saudi Arabia, Bahrain, Qatar, Oman, United Arab Emirates, Jordan, Kuwait, Morocco, Cambodia, Brunei, Tonga and Lesotho.