If you’re a longtime user of Adobe products, you’re probably accustomed to using Command-Spacebar (PC: Control-Spacebar) to zoom in. It works in Photoshop, Illustrator, InDesign and other apps.
But it seems (initially) to be broken in Acrobat X. Since you’re already traumatized by the radically-changed interface in Acrobat X, you may just assume that the old zoom shortcut is broken. You press Command+Spacebar, and nothing happens. You just figure, well, it was fun while it lasted.
All is not lost, however; you just have to use a bit of finesse. It’s a one-two punch: press and hold the Spacebar first, then — a half-second later — press and hold the Command or Control key. Voilá (which is French for “Zoom tool”), you can now click and zoom. It’s still the same combination of keys; you just have to press them in order (then hold) rather than simultaneously.
And no, I don’t know why. It’s one of those Great Mysteries.