As Tom Francis pointed out last weekend, one of the reasons The Phantom Pain is an enjoyable stealth game is that it's very forgiving of slip-ups. If you're a little careless and draw the attention of a guard, you're given a number of opportunities to wriggle your way out of your mistake.
On the other hand, not everyone is looking for a game that bends over backwards to forgive screw-ups, and I know a lot of people play with reflex mode off, simply because they feel it's too easy. If you're someone looking for more of a challenge in MGS 5, modder JRavens has you covered. The Phantom Pain Hardcore mod turns up the difficulty with a number of tweaks.
First and foremost, enemy hearing and vision has been improve by around 50%, meaning they can detect you at much greater ranges now. I tested this, and it definitely feels like a big change. While creeping up on an enemy checkpoint, I raised the suspicions of a guard while I was crawling, still a good distance off, and partially concealed by a boulder. The distance at which you can be detected is also randomized, meaning you'll never be quite sure how many meters away you need to stay to completely avoid being spotted. Soldiers are now also better at detecting when you've Fulton extracted one of their buddies.
Enemy knockout time has been decreased as well. I didn't really test this one for myself, because after tranquilizing a guard I got bored waiting to see how long it would take him to wake up. Bullet damage, however, I did test, and it does appear bullets hurt a heck of a lot more. I feel like Snake is typically pretty durable, and things only get really dire after long, extended bouts of being shot, but I entered the danger-of-dying zone pretty quickly when some soldiers opened up on me.
You're not the only one more susceptible to bullets. If you call in air support, you best be ready to help your chopper take down enemy soldiers, because it is now highly vulnerable to small arms fire. Can confirm: I called in my chopper (blasting Weird Science, of course) and it immediately began to take damage from a handful of soldiers who weren't even using antiaircraft weapons. Then it blew up.
Soldiers, meanwhile, have had their limb health improved, so don't expect them to fall down unless you put a couple extra rounds in their legs. Speaking of Snake's ammo supply, it's been punished as well, and he can now only carry about half as much as he normally does. And that empty magazine he could throw around to distract guards? It's no longer infinite. You can use it six times and then you'll need to call in a supply drop if you want more.
All of this should provide a more hazardous environment for those of you who want to put your skills to the test, or who at least want to put poor Snake through the wringer. You can find the mod here at Nexus,—keep in mind JRavens is still testing and tweaking it—and there are installation instructions in the description and the download.