Most of it appears to be the introduction of something they call Creation Club. Also some new equipment that is mostly new armor and a couple of new weapons. Improved modification support. Support for .ESL files.
"Featuring new items, abilities and gameplay created by Bethesda Games Studios and outside development partners – including the best community creators – Creation Club content is fully curated and compatible with the main game, official add-ons, and achievements. Currently available for Fallout 4, Creation Club will be coming to Skyrim Special Edition next month. All Creation Club content works on PlayStation 4, Xbox One and PC. And of course, Mods will remain a free and open system where anyone can create and share what they’d like. "
She was grimacing. "That does sound like what America's has been trying to do for the last century or two--get rich faster than the parasites could steal it." The Free Lunch by Spider Robinson