Have no fear, Twitter Inc. should be worth piles of money when it sells shares to the public. Back in February, my back-of-the-envelope guess was that it could be valued at more than $12 billion, a level it will probably easily beat.

Having spent a week with its 120,000-word IPO document, I'm left with a nagging feeling. The document is full of numbers and charts and graphs that show how the service is growing. But if you look hard enough—heck, if you look at all—you'll see it has virtually no details about the most important...