Notable features include non-Steam game compatibility for Proton, ext4 and exFAT file system support for the SD cards, user-accessible BIOS with multi-boot option along with SD card OS boot, plus PC game controller mode through Remote Play. The updated FAQ also confirms that external GPUs are not supported and the console is not specifically optimized for VR games.
https://www.notebookcheck.net/Valve-reveals-more-Steam-Deck-features-in-updated-FAQ.564290.0.html