Although Gutenberg is justly famous for being the inventor of the Printing Press, the invention itself is grossly misnamed in that appellation. What Gutenberg actually invented, and which changed the world, is the Movable- And Reusable-Type Printing Press.
By devising a system in which mass-produced lead type, cheaply cast in large quantities, could be rapidly set; used for printing; and then disassembled to await the next print job, Gutenberg revolutionized how literature was disseminated.
Prior mechanisms involved the painstaking creation of block text, specific to each print job by master craftsmen. In Gutenberg's new system, only the molds for the typed letters needed to be made by master craftsmen; the type itself could be made by less skilled journeymen.
The speed difference arose from the availability of vast quantities of type, in various font families and sizes, in local print shops. Setting this type could be done by a skilled typesetter at perhaps a quarter the speed of reading it. The actual print runs were comparable, but the typeset was available in hours rather than weeks or months.
The simpler Roman alphabet, compared to those of China and Japan, certainly would have been a motivating factor for Gutenberg's invention, but perhaps simply made the entire notion imaginable. One can start with 52 upper- and lower-case letter, 10 digits, and a few punctuation marks, totaling about 70 molds, compared to a basic vocabulary in Oriental characters of several hundred to a few thousand starting characters.
As noted below moveable-type existed in China for a few hundred years before Gutenberg, but the type characters were individually fired porcelain, thus much more expensive to make than Gutenberg's molded lead type, and not practical for large-scale printing. This reduced their effect on the cost of a book, according to the economic analysis presented by Lennart.