I'm not 100% on my update dates but I'm pretty sure it's almost a different game. If you were still around when the first open world on Earth dropped, there are now 2 more on Venus and Deimos, and a third in the form of Duviri, which takes place adjacent to the main story. If you weren't around then... well, there are open world areas now.
Empyrean brought in Railjacks, space fighters you can crew with 3 other tenno (or a group of NPCs) in specialised missions. It's... OK. fun for a while, but arduous to grind, even by Warframe standards.
There are personal bosses in the form of Grineer Kuva Liches and Corpus Sisters of Parvos, which can be killed through an elaborate game of mastermind for their shiny weapons or taken possession of so they'll fight for you in battle.
The New War gives the Sentients much heavier involvement generally, and features a couple of really fantastic story missions. With the sentients comes the Archon Hunts, which are sorties but harder.
The Void has gotten more involved during several updates, giving a new class of top tier weapon modifications, a new extremely-to-obscenely-high level content mode, and an endless mission mode.

On top of the content, there's been a lot of streamlining, UI modification, remastering of old maps, reworks of frames, reworks of Railjacks, reworks of Archwings, a damage rework, a syndicate rework, and that chair that pops Nidus' cysts in the back of your orbiter now does more stuff.

I haven't been on since 1999 dropped, but last time I checked it was in the best state it had ever been.