Wood wouldn't do; each letter would have to be hand-carved. Gutenberg poured metal into molds to form the letters; the only limit on the number of letters (to be formed into limitless combinations in pages) was the amount of metal; but the kind of metal remained a question. Lead was too soft, and the pressure of the printing press would flatten it. Iron was too hard and heavy, shredding paper and requiring high levels of heat. Bronze was expensive. Gutenberg, after much experimentation, settled on a mixture of antimony, lead and tin. It's still called printer's metal to this day.
Printing eventually would revolutionize education and society, bringing literacy (and social unrest) to the masses through cheap, mass-produced reading material. Until then, printers had to be near their customers, the literate class. These were the university professors, the clergy, the lawyers and the nobility. That meant setting up shop in the heart of the city, where waterpower was unavailable and wind power unlikely. Pressing paper to type took muscle power, and Gutenberg adapted a screw press, perhaps one used by winemakers to squeeze grapes. Still, repeated raisings and lowerings of the screw were hard work, and a printer did well to turn out 200 pages a day, one page at a time, on his wooden flatbed press. Nevertheless, it was a vast improvement over printing as it was then practiced: rubbing ink onto carved woodblocks and then pressing paper against the blocks.
Not all inks are created equal. Ink that is suitable for woodcuts won't work well on metal type, as the scholar S.H. Steinberg points out in his book "Five Hundred Years of Printing." Water-based inks dribble off printing surfaces and bleed into paper. Not all kinds of paper are suitable to printing, either. Early printers jealously guarded their ink recipes, usually a formula of lampblack (the soot from lamps) and oil, which made the ink just sticky enough to adhere to the printing surface -- the type -- but not too sticky, so it would transfer to the paper under pressure without spreading. Paper was made from rags, and sized, or coated. Thus the ink would adhere without soaking into the paper.
Gutenberg's principles would not be improved upon until photography and electronics were applied to printing, but mechanical improvements tracked the growth of the Industrial Revolution that Gutenberg helped bring about. Iron and steel replaced wooden presses. Steam power, then electricity, were applied to run presses, which later had cylinders to hold and press paper. Wood pulp made paper, later formed in rolls and not sheets. Fountains applied the ink that printers once rubbed onto type by hand; typesetters generated shiny lines of type, where compositors once formed words, one letter at a time, by hand.