Detachment be damned!
One of my biggest problems with MMO combat is how absolutely detached it can be. Clicking your target, hitting your auto-attack button, and then splashing a few skills into the mix makes for interesting tactical decisions, but sometimes it doesn't feel like you're there. It feels like you're watching your character have an adventure, not like you're having the adventure.
The first person perspective very quickly eliminates that barrier between the on-screen character and behind-the-keyboard player. No longer are you detached and floating from behind, but you're right in the character's head, seeing things the way they see them. Now you're the one moving around, avoiding the monsters, and setting yourself up for the right attack at the right time. You're the one hefting the axe and bringing it down on some unsuspecting guy's back, not watching as your character on screen screws up the sneak attack against enemies who know your every move.
Touch someone from afar: Use your sights
When you add the first person perspective to gunplay, you almost always have the option of looking down the scope of your gun. While not everyone loves being a sniper, sometimes you just want to get some magnification on your next shot, so you can be sure it doesn't miss.
Fallen Earth utilizes these awesome modes as well, giving you the edge in being able to select your targets from a distance. Why jump into combat and get up close and personal when you can remove someone's head from the window of a run down building? Very few games let you have these gun-and-run options, where you can take out a few enemies and get away before they even knew what happened.
As I said above, FPS combat makes combat exciting. It's no longer a point-click-and-wait exercise of patience. Now it's a game of movement, aiming, and positioning as you use your skills (the ability to aim and move) along with your skills (the buttons on the hot bar) to keep yourself alive.
Above all, the combat fits with the theme of the apocalypse. Sitting and waiting for your character to beat the crap out of a rabid mutant just doesn't work when you compare it to pulling out a pistol and aiming at someone's leg. It's a setting that craves action, and the developers at Fallen Earth knew that all too well.