You could argue that with more on-board storage the Lumia 920 doesn't really need an expansion card slot, but Nokia executive vice-president Kevin Shields says the reason is far simpler than that: Nokia didn't want to “defile” the phone's design.
Speaking to PC Pro, Shields claimed that from the start Nokia wanted to keep the Lumia 920's design as clean and uncluttered as possible, and so decided to do without the card slot.
“We started with the premise that we wanted an uncompromised physical form,” Shields explained. “To put an SD card slot in it would have defiled it.”
He also argued that the 32GB of internal storage on board the Lumia 920 (versus the Lumia 820's 8GB) was sufficient for the vast majority of users – or as he put it: “most people aren't using the storage in their phone, anyway.”
What about at least giving us the choice, Kevin? Sure, most people don't need more than 32GB, but those that do are typically early-adopting, media-consuming power users who know a lot about phones and are quite likely to make buying decisions based on exactly this sort of issue.
And returning to the design argument, maybe the comment about “defiling” the Lumia 920's form was made slightly in jest, because if not it suggests a poor understanding of what good design is all about.
Good design isn't just about making a product look good, it's about a combination of form and function where one complements the other. For the most part the Lumia 920 does achieve that, but leaving out useful features essentially just because you don't want to be able to see them is flawed thinking right from the start.