This is not a full list of features, some are surprises or mysteries. These are good to read to be prepared for the dangers of Journey UHC!

Core Features:

• Ultra Hardcore mode: no health regeneration. Extra lives can be gained, up to 2, by eating Enchanted Golden Apples. If you die without extra lives, you can't join the server anymore. There are two ways to get new lives at that point: via a Player Anchor or from the JUHC Life Challenge. Other than that, your only option is to have your playerdata reset on request.
• No true logout: your avatar will always stay on the server, even when you log out. By sleeping in a bed for 15 seconds, you log out safely. This makes you invulnerable and hides your nametag until you log back in, but you'll still be vulnerable to traps if you don't sleep in a safe location!
• Random Spawning: there is no global world spawn point like in normal worlds. When you join initially, you spawn randomly in the 100.000 by 100.000 block world. When you respawn (after death or when returning from the End) you will spawn randomly in a 5000 by 5000 block area centered on your last position in the overworld. The End is still vanilla, so expect to run into other players there!
• Distance-based chat: only players within render distance can read your chat messages. You can't even see who is online! You truly never know what's out there...

Changes:

• No Coordinates: there are no coordinates in the f3 menu. Use Maps, Compasses and Lodestones to find your way.
• More danger: all common hostile mobs have received speed boosts and will spawn with equipment more frequently. In particular, watch out for these:
  • Phantoms are much faster and can knock you out of the sky when flying with Elytra.
  • Drowned are much faster, much more common, and will break your boat. The ocean is now an extremely dangerous place, especially at night!
• Master Ender Chests: just like how your avatar is never truly safe, your Ender Chest isn't either. To be able to use Ender Chests, you first have to create a "Master Ender Chest" somewhere in the End, by using a Nether Star on an Ender Chest.
• Mending nerf: in vanilla, once you have access to the Mending enchantment, you basically never have to work on your tools again. That would be too easy here, of course. I haven't removed Mending, but it no longer repairs your tool - instead it slowly lowers the cost of repairing your tool in an anvil. In vanilla, repairing will become "too expensive" after a few times, so in this way, Mending still allows you to repair your tool indefinitely, while also adding a material cost.
• Elytra nerf: Minecraft's other really overpowered feature is the Elytra and Firework Rockets. Using Fireworks while gliding now does some damage, and being hurt by an external source while flying will temporarily disable your Elytra, causing you to potentially fall to your death. So use these wings with caution!
• Nutrients: food is split into 3 categories: protein (meats), vitamin (fruits and vegetables) and sugar (processed foods). Alternate food sources to prevent health issues:
  • Protein level affects your sprinting speed
  • Vitamin level affects your damage resistance
  • Sugar level affects your happiness (coming soon)
• Suspicious Stew can't be crafted, it is instead made by adding the flower to Mushroom Stew within a Brewing Stand.
• A new Ender Dragon will occasionally come to the central island, so always be prepared to fight one when going there...

Additions:

• The Happiness System: I'm leaving this one as a bit of a mystery for you to figure out. High Happiness has both advantages and drawbacks...
• Player Anchor: Want to play with your friends? Find a Zombie holding a Totem of Undying and give them an Echo Shard to create a special kind of Zombie, called a Player Anchor (recognized by soul particles). Then, use a Name Tag to name the Player Anchor after your friend's exact username, before they log in for the first time. Now when a player with that username joins for the first time, they will be given the option to spawn at your Anchor instead of randomly.
  Alternatively, you may want to bring another player back to life. Dead players can also join the server if you create a Player Anchor with their username, but they can then only stay for 5 minutes - unless they eat an Enchanted Golden Apple to gain a life within that time.
• Ear of Ender: hold this device while sending a chat message, and your message will be received by all players who have an Ear of Ender with the same name in their inventory. Recipe:
  
• Thorny Skeletons: These might be a nasty surprise! They can be recognized by their crit particles...
• Beacon spawnproofing: Put a Mob Head on a beacon and kill a bunch of that mob type near it!
• Witch Heads: behave the same way as other mob heads.
• Map features: any Map can be right-clicked to swap with any Map in your inventory that contains you, if such a Map exists.
• Map Atlases: these useful items are found in bookshelves in two existing structures... you can figure out how they work :)
• More things that are secrets for now...

We're playing on a Fabric server, which means that any vanilla contraptions that aren't affected by any of these features should work the way you're used to.

Future planned features:

• Resetting your data: you can contact me to do this for now!
• Custom structures: special challenges with unique items as a risk/reward system
• Protector block: a way to make it hard to break your builds
• Mobs that break blocks which attack advanced players occasionally
• More endgame content in the far reaches of the End