Bethesda has been teasing this Survival Mode update for a couple of months now, and from what I'm hearing on various forums it was well worth the wait as it turns the casual stroll through the Commonwealth in to a true struggle for survival where even the smallest of wounds can spell doom.
If you would like to give it a try for yourself and see how long it takes before diseases strikes you down, Survival Mode Beta is currently available on Steam, and it sports an impressive list of changes and additions. If you're looking for detailed descriptions head over to the Bethesda website, but if its a short overview that you seek, I've prepared one:
Saving with sleep: You can no longer save the game normally. In order to save your game in Survival Mode you will need to find a bed and sleep for at least an hour.
No more fast travel: You can no longer fast travel via the map. Faction options such as vertibirds or teleportation still work perfectly fine.
Increased lethality: Both you and your enemies take, and deal a lot more damage. Your damage output is then even further increased through Adrenaline.
Adrenaline: As you kill enemies you will periodically gain stacks of the Adrenaline buff which increases your damage by 5% for each stack. Sleeping will remove either a couple, or all of your Adrenaline depending on how long you decided to sleep for.
The compass has been nerfed: Enemies will no longer display on the compass and the distance at which locations of interest appear on your HUD has been significantly shortened.
Wellness - exhaustion, hunger and thirst: As you might imagine from the name, you will need to rest when you're tired, drink when you're thirsty, and eat when you're hungry. In order to make this new system a lot more interesting Bethesda is also adding items that play around with your wellness stats, so some will make you thirstier in exchange for buffs, others more tired and so forth.
Fatigue: Fatigue mostly comes from being tired, but its exaggerated by hunger and thirst. The more fatigued you are, the less AP you will have for V.A.T.S and sprinting, so make sure you rest every so often or you might just find yourself crawling everywhere.
Bed types matter: The better the bed you sleep on, the higher the amounts of health you will recover.
Illness & antibiotics: You can now get sick if your wellness is low enough, or if you partake in some sketchy habits *cough* JET *cough*. If you do get sick you will need to either craft an antibiotic, buy one from the various doctors around the world, or ask them to heal your current illness for a fee.
Cleansing radiation comes at a cost: Items that cleans your body of radiation will make you tired and your immune system weakened, which means you will be quite prone to illness. In other words, don't chug RadAway unless you're in the safety of one of your settlements.
Slower healing: Healing items such as food or Stimpacks will now recover your health at a significantly slower rate.
Crippled limbs: Limbs will no longer auto-heal after combat. They will remain crippled until you either heal them with a Stimpack, or until you take a nice, relaxing nap (I wish it worked that way in real life as well).
Carry weight: Your maximum carry weight has been reduced and items such as ammo and Stimpacks now have weight as well. Chose what you carry carefully, or invest in a whole lot of strength.
Companion costs: Companions can now carry less weight and they will no longer automatically get back up after combat, you will need to Stimpack them to life or they will get fussy and go home.
Enemy and loot repopulation: Enemies and loot will now respawn at a significantly slower rate, so don't expect to find a loot cave in this one.
And there you have it, that's the Fallout 4 Survival Mode in all of its hardcore glory. If you encounter some issues while playing it make sure to send feedback to Bethesda via their forums.