I’ve played both, and they’re fundamentally different kinds of games. Warframe is all about movement, mixing shooting with melee, and collecting mods and frame parts to boost your utility and lethality. D2 is more of a standard FPS with class-specific magic abilities.
D2 has a decent intro level to get you up to speed with how to play, but you don’t really grasp what Warframe is about for maybe 20-50 hours, and you don’t grasp the meta until maybe 100-500. There’s a lot going on. Joining a clan is an absolute necessity, because that unlocks pretty much the rest of the game for you.
I had a fun time with it, and it’s probably one of the best F2P models out there, but I don’t have any plans to ever pick it up again.