The Final Temptation: Batman was caught in a virtual dream world by the Mad Hatter (who fortunately could not observe the dream itself) living a life as playboy billionaire Bruce Wayne, with his parents alive and well (and thus he had never become Batman). In the dream he was engaged to Selina Kyle (who was not Catwoman) and the mysterious Batman was someone ELSE who just showed up in Gotham. His friends and family in the dream almost had him completely convinced he had created his real life as a delusion to escape his feelings of not having accomplished anything on his own, but the illusion fell apart when he discovered he couldn't read anything (because reading requires parts of the brain not used in dreams?). Suffice to say, he did NOT appreciate the Mind Screw when he woke himself up.
Trying to read something in a dream is a trick used by lucid dreamers to detect whether they are dreaming. Unless you have a photographic memory, your brain will have some trouble trying to remember or imagine a page full of text.
Foreshadowing: During the first appearance of "Batman", one of the storefronts in the background has a sign on it that's illegible gibberish, it could be just an artifact of being drawn by the animators as filler, but, in retrospect, it could be a subtle clue as well.
Homage: Gordon says to Batman "Any idea what it [the mind-control helmet] is?" and Batman deadpans back the final line of The Maltese Falcon:
Batman: Yes. The stuff that dreams are made of.
That line is also an homage to The Tempest Act 4, scene 1
Pinch Me: Bruce realizes it's All Just a Dream when he finds he can't read any books (based on the idea that the left side of the brain, which controls the interpretation of written words, is mostly dormant during dreaming), though it takes him a bit longer to figure out how to free himself.
Your Mind Makes It Real: Bruce gets out of the dream state unharmed by "dying" in a fall, betting against the trope. Judging by Mad Hatter's reaction, this is the only way to get "out", and he's shocked someone would willingly end what should be their greatest fantasy.