What is the catch to this game being F2P?
Everything you want in this game in terms of new warframes and equipment comes from raw materials you have to gather from drops you collect during missions, which all feed into a rather sprawling crafting system with real-time construction times. A small item can take 60 seconds to craft, a warframe may take a few days.
You can make basic items from more common materials provided you have the blueprints (bought on the market with in-game credits, or found in missions), but to make the more specialty gear you need rarer items found only from boss drops or high end missions.
Platinum is the real-money resource for Warframe. You will never ever get platinum in-game, but you can get almost everything that platinum is used to buy. With platinum you can rush the construction of items, or buy them outright. You can also buy more equipment slots, visual customizations and things like clan images.
So the game is one huge crafting grind. But the missions themselves are fun hack and shoot-fests with friends or solo. Its got jank, but I'm willing to forgive it when wallrunning, flipping, and sliding into a 360 glaive attack followed by an explosive charge, following it up with headshots. Its something else to be a space ninja in this game.
I havent felt compelled to spend any platinum on shortcuts and don't think I ever will, but I have paid DE some monies and used it on customizations. I also picked up a glaive because I didn't know you could actually find one (rare but do-able), and that thing rocks.