The demo has been out since 18 August, and a lot has piled up in those two weeks. Almost everything here comes straight from your reports in the forum or from co-op test evenings with two accounts. Thanks for those, especially the detailed ones.
Startup and frame rate
The first launch stuttered for about eight seconds while inventory icons were being generated in the background. That work now happens before you reach the menu. It was the very first impression the game made, and the worst one.
The menu that would not go away
On the very first launch, an invisible copy of the main menu could stay on top of the running game. It swallowed your clicks, which is why the game looked frozen or unplayable.
The same copy also showed your save slots as permanently empty, because its list was built before you had ever saved. If that made you think the game wasn't saving: your saves were fine the whole time.
It was triggered by the one-time language prompt, so it only ever happened on the very first launch and was gone after a restart.
Brightness
Night and dusk were too dark to play in. This was the most common point in the entire forum.
The darkest moment turned out not to be night but sunset: there is a window after the sun has gone down and before the moon is properly up where almost nothing lights the deck. That is exactly when you are tidying up and building your first lamp.
Unlit surfaces now have a light floor. Daytime looks the same as before, and night stays night.
There is also a brightness slider under Options, Display. Screens differ, and nobody should have to live with my setting.
Playing together
This is where most of it was broken, and you only ever see it when a second player is watching.
"Back to main menu" did not actually leave the session. Starting a new game dropped you back into the old one, and only quitting the game completely helped. The guest was never told either, and kept building in a world that no longer existed.
After a session ended, the other player's character stayed standing in the world, even in a freshly started single player game.
Anyone who joined did not wake up on the host's raft, but where the host's world had once started. The further the host had sailed, the further out at sea you appeared.
The guest could not pick up the five pieces of driftwood you start with. The first task in the game was impossible for anyone who joined.
Everything the guest picked up left a copy behind that kept drifting on the host's screen.
Tools sat wrong in the other player's hands, and the bow, fishing rod and diving goggles were not shown at all.
As soon as someone had swung a tool once, their character faced sideways for everyone else.
With the Steam overlay switched off, the invite button could not open anything and said nothing about it. It now explains why before you click, and points out that a friend can join through the Steam friends list instead.
Small things you notice
A rope ran aft from the sail and ended in mid air. Same on the trader's boat.
The fishing rod had a loose piece of line floating next to it instead of attached.
Campfire smoke was invisible in front of the water and only reappeared above the horizon.
A railing could not be placed on the edge of the raft even though there was a foundation under it.
The spear now thrusts forward instead of just wobbling.
Known, still open
The crosshair is hard to see against bright water.
Inventory items cannot be rearranged by hand.
One thing that keeps coming up: co-op is fully in the demo, nothing is cut. The invite button only appears once you have opened a world, not in the main menu.
If something still does not work, post it in the discussions. It gets read.