The free version offers enough gameplay potential that you're easily able to decide if it's fun enough, for you, to pay for. You don't quickly run into paywalls, so there's no feeling that you're missing out on something major without subscribing, though you'll want to if you play constantly.
Fallen Earth is set in the year 2062, in an area of the Grand Canyon. There's no attempt to portray the game's setting as a model of the entire world, or even of North America; it's all a continuous region, with the hazards and rewards increasing as you travel around.
It's a free-form setting; Fallen Earth doesn't channeled into the "right" areas, although there are quests to follow if you want. You are free to wander into places where the dangers are more than you can take. New regions are added slowly, but regularly, and Fallen Earth has a small number of large regions with different types of terrain within them, instead of a large number of small, but distinctly flavored, "zones".
Combat in Fallen Earth relies on a mix of player aiming, character skills and game statistics. The player constantly flips between combat mode, where the mouse aims the reticle (the crosshair placed in the eyepiece of the scope), and non-combat mode, where you click on objects, activate interface buttons, and so on. Skill gain comes from using skills, and also from attribute points, which can be more freely spent as you earn them. There are no classes; you can mix-and-match skills as you wish, but there aren't enough points to be good at everything, and a jack of all trades is a master of being mutant-chow.
Crafting is a major aspect of Fallen Earth. Virtually all items are player-crafted, and gathering from resources, from "tainted water" to "scrap steel". Part of the richness of the system is shown by the fact there's more than one way to skin a mutant chicken: You can get scrap steel from harvesting appropriate ruins, or you can make it from scrap iron and coal.