First things first: There is alot of free content in this game and i mean A LOT. Many mission types and a plethora of kind of activities(relic openings, eidolon hunts, sorties, circuit, etc). The game has even a story, which in fact, is amazing for a free game. Every new story addition is free and you can play it from the get go.

You have so many warframes, weapons and builds to choose from, you can customize them all by the way. You can't get bored with it. I remember I was in my final year of highschool and I played warframe up to 12 hours in a day and I still had things to do.

Also, if you suffer of FEAR OF MISSING OUT, dont be scared, you can always quit and come back. This game right now has set some things right, we finally have an end game.

Warframe offers a vasts options for traveling, you have your ship which you can travel and do missions with other 4 people in it, as in Star Wars. If you want to travel the open worlds(yes, this game has 3 open worlds as so far) you can choose from your k-drive or arch-wing. You can own pets, or drones that go with you in your missions and help you(and in fact, arent useless at all at the moment).

What else can I say? Play it, best F2P experience out there.