While the company made progress in 2012, with its net loss narrowing by 38 percent, this year its net loss increased by 41 percent, as it has dramatically stepped up its acquisitions.
The big question is when will those acquisitions start to boost Twitter's bottom line?
Going into the filing, a number of potential investors said they were curious to see how Twitter's revenue per user would stack up toFacebook's—it's less than half. In the second quarter, Facebook's revenue per user was about $1.60, while Twitter's was only $1.18 for the entire first half.
2) Has user growth stagnated?
Twitter's 215 million user number is surprising; it shows the company only added 15 million users in six months. That's particularly disappointing considering it was reported a few months ago that it had as many as 240 million users, and heading into the IPO, it was widely expected that the company's user base was growing fast. It's not. And in comparison to Facebook's 815 million users when it filed, Twitter is small.
3) International opportunity AND challenge?
Twitter has a remarkably diverse user base—more than three quarters of its user base last quarter was outside the U.S. However, only a quarter of its revenue comes from those users, which can be seen as a big problem as well as a big opportunity.
4) Focus on traditional media
In its S-1, Twitter says it's eager to partner with traditional media companies to bring more content to Twitter. Interestingly, the company says that it competes with traditional media for ad revenue as well as talent. Twitter's deals with CBS and the National Football League last week were meaningful, but it'll be interesting to see how much revenue they drive.
5) An UNUSUAL single class of stock?
Media companies generally issue two kinds of stock—for the public, and "preferred shares," which have far more voting power and are owned by the people who run the company. Facebook, for instance, is a "controlled" company—by Mark Zuckerberg. In sharp contrast, Twitter has just one class of shares, so all shareholders have an equal vote. That means CEO Dick Costolo, and the largest shareholder Evan Williams, won't be able to exercise outsized control of the company. Twitter could still issue preferred stock, but as of now, it is giving shareholders the same kind of equal voice that Twitter users have now.
—By CNBC's Julia Boorstin. Follow her on Twitter: @JBoorstin.