So I tuned back into this for the moment. Several incremental things.
You can have a Kubrow pet now. They look ridiculous.
New Warframe is Mirage. Something to do with Light/Shadow. Their first ability is some sort of super broken passive depending on what lighting they're standing in.
Total UI overhaul. It's terrible. From the planetary system map to the Arsenal page, it's pretty awful. It's like the UI artist just went "Fine then, FUCK YOU" and made it all white boxes. And who can blame them? This is like the 5 or 6th UI they've tried, and the current UI is a skeleton of bits and pieces people didn't hate.
That said, one part I like is that now a lot of menus are done in screen, so your Tenno projects the options menu in front of them and their head follows your mouse cursor. That effect is pretty cool, until something gets cut off on the screen.
The whole player hub has become a thing now instead of just a screen now too. You get your own fuckin' ship, man. Which a friend of mine dubbed the Hate Chamber because it's so tiny and you can tersely pace around in it like you're pissed off. Your Arsenal, mods, codex, galactic map, foundry and market are all there in this tiny 1 Tenno ship, complete with an AI you wish would put a sock in it. There's a new tutorial that shows off all this and gives a proper story-like introduction to being a Tenno. Grineer bad, Grineer capture Tenno, Tenno kill all Grineer, something something tech something something 2 missions something something DARVO something something missions something something Boss Fight. Nothing you don't already know.
Lots of audio changes to core SFX and stuff.
Few other changes to Dark Sectors ect...but I haven't read too deeply into it.
If you haven't played WF in a few months, it will look dramatically different than the last time you saw it.