Everything bad about Superman for the next 30 years starts here!
Thirty issues into Superman, the Man of Steel had gone from a guy who was able to leap tall buildings in a single bound - to a guy with nearly godlike powers. How do you find someone for him to fight? The writers finally hit on the answer with this issue - have him meet a guy that actually does have God-like powers.
Enter Mr. Mxyztplk, the answer to every lazy writer's dreams, the 'holodeck', if you will, of the Superman series; a cartoon-y character from another dimension who can do anything with a mere thought. And now we have the ultimate stalemate - Superman will not be about challenging adversaries, but jokes and gimmick stories from now until the 70s.
So how does Superman defeat Mxyztplk? He has to trick him into saying his own name backwards (a pitifully easy thing to do - apparently you don't have to be too bright if you're omnipotent) - so, yes, you guessed it, from now onward Mxyztplk will show up from time to time, do a bunch of stuff that Superman is powerless to prevent, Superman will run around doing damage control for 6 pages, and then he will find a way to make Mxyztplk say his name backwards and disappear for another few months.
Whatever happened to the Man of Tomorrow? This did.