CFO to CEO - What does it take for a CFO to become CEO?

My quick take; it depends on what the board is looking for, and what the company does.
If the company is in Mfg, then having line experience is critical.
If the company is in a competitive marketplace, then running Sales / Marketing is critical.
If the company is in cost-cut "milk the cow" mode, then having been a CFO is critical.

When Intel made Paul CEO (granting that he'd not been a CFO, but had originated in the finance ranks), his experience running Intel's Sales & Marketing group was seen as critical as Intel's problem wasn't making chips efficiently, it was that the market was outstripping the technology. (disclaimer: I worked at Intel, but was not even vaguely in the loop on this decision, and the former is personal opinion)

One thing that can trip a CFO is when the CFO is viewed by peers/CEO/Board as being the "bad cop" to the outgoing CEO's "good cop". It is difficult to transition from enforcer to leader, as they can appear to be very different roles. Having a strong Corporate Controller or VP.F. take the enforcement role can let you transition your image and relationships to a more leadership tack.

The CFO's reputation can be a market signal if appointed permanent CEO; what does the market think of the CFO? Your personal brand can influence how the finance community sees the future of the company.

The ability to create a vision, execute a strategy, and deliver real -measurable- results. CEOs are not numbers nerds, they are thought leaders (possessing soft skills) capable of driving vision / results.

From someone who has made the transition. The successful move from CFO to CEO will depend on the organization and their view of the CFO. If that role is that of a trusted advisor and business partner rather than the numbers guy, the transition inside an organization is easier to make and easier for others in the company to accept. Otherwise, take the big step and start your own business.