How common were prequels before The Phantom Menace?
It seems ever since in movies and TV we've seen tons of prequels. Not much of it being very good.
Smallville. Gotham. Enterprise. The Hobbit Trilogy. The new X-Men movies. The Thing prequel. Oz the Great and Powerful. Prometheus. Even the Alien vs. Predator movies could be considered prequels to Alien.
Out of that list I did like X-Men Days of Future Past. Not much else.