Someone else has already had the same thought about IOU as I did, so I’ll make this brief:
I-O-U = Iodine, Oxygen, Uranium (Sherlock realizes this in the lab with Molly, when he’s looking into the microscope and mutters “IOU” under his breath) = atomic numbers 53, 8, 92 = corresponds to something in Grimm’s Fairy Tales.
That was as far as I was able to get, because I couldn’t find the same edition of Grimm’s Fairytales that Sherlock has in the show. Thankfully, someone else on Tumblr had the same thought, found the correct edition, and figured out that fairytales 53, 8, and 92 in that version are Snow White, The Strange Musician, and King of Golden Mountain.
The fairytales correspond to the things Moriarty “opens” with his fake key (the Crown Jewels, the Prison, and the Bank of England vaults). They also match up with other themes and events throughout the episode.
Snow White = Crown Jewels
Moriarty poses himself as the Evil Queen, tempting Snow White. Later, he offers Sherlock an apple. Sherlock appears to die, like Snow White, and then comes back to life, also like Snow White.
Strange Musician = Prison
In the story, a violinist traps a fox, a wolf, and a hare while he’s out searching for a companion. By the end of the story, the fox frees himself, then frees the wolf and the hare, and the animals hunt the violinist. Note that when Moriarty visits Sherlock for tea, he’s wearing a fox pin on his lapel.
King of Golden Mountain = Bank of England
Obvious correspondence there, just in name alone. In the story, a boy takes a hero’s journey, overcoming obstacles and killing his competition in order to become king. “The man with the key is king.” This is a theme that’s almost heavy-handed throughout the entire series.