Yep, that’s pretty much it. The grind + unique movement system is the game, and maybe the appeal isn’t for everyone. I had a clan mate who came from D2, got maybe 800 hours into Warframe, but ultimately would always go back to D2.
The grindy pieces can be fun, especially with the amount of meta builds to make things easier (like Eidolon fights), but a bad team comp can make a mission harder (via a vis the medallion thing you’re talking about).
I had a hard time in the beginning, and I put the game down several times, too. I agree that it can be very confusing. And yet, it still called me, and I wound up putting in 1000 more hours; having a very active guild helped with that, too.
But if it’s not for you, that’s totally fine. It’s visually very pretty, and the fact that you can earn premium currency with some sweat equity is unheard of in F2P games, but you are right that how combat works is pretty much the same for every mission. What makes things different is the frames and mods.