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
Finally, They Addressed The Add Game Feature! Nice.
I was assuming there was no thunderbolt but I would have paid more for that as it would have been the best of both worlds if you had a GPU dock with keyboard, mouse 4k 144hz monitor all ready to go at home. Perhaps even a VR setup. One system for both purposes.