It doesn't take place in the show's area, it takes place on the west coast around a destroyed San Francisco. There are a few story missions featuring characters from the show, but mostly its just set in the same universe. Gunplay is pretty similar to borderlands except in a 3rd person perspective and more of a focus on perks and powers. I just owned the base game on PC (none of the DLC), and played it for about 100 hours before I lost interest. You can play a lot of the DLC missions/raids without owning the DLC, though half the time they give weapons as rewards that require DLC to equip.
Enemies match their HP and difficulty based on the level of you and the other player's in their immediately vicinity. So there is no wandering into a new area and getting slaughtered by impossible enemies, you pretty much have your run of the whole map from the start, though some areas feature enemy types that are still fairly difficult.
The perk system is there is a grid of squares divided into 4 quadrants. In the center of a quadrant is one of 4 primary skills. Overcharge which boosts damage temporarily, Cloak, which gives you the ability to go invisible temporarily. Blur which gives you a short term boost of speed, and decoy which lets you create a hologram to distract enemies. Once you pick a primary skill on the next level up you can either level up the primary skill further or choose to unlock any other perk that is touching an already unlocked perk. Perks in that skill's quadrant usually buff the primary skill in some way.
You can only have so many perks equipped at a time (get more perk slots as you level) as well as create different loadouts with different weapons and perk permutations that you can switch to on the fly. If you level up far enough you can inch your way over to another quadrant and unlock another primary skill (though only one can be active at a time). So at one moment you could be a stealthy rogue cloaking to get a sniper shot to the back for extra damage, and with a click in the menu, now be rushing in with blur to shotgun and melee the enemy to death. You can also respec at the cost of in-game currency, though it gets more expensive the higher you level up.