Here is a list of what Minecraft needs in Multiplayer and in Singleplayer. It is a brain dump of ideas. Couldn't sleep because of it.

1. In multiplayer, chests need passwords so people don't grief and steal everything.
2. The chests need to have name plates on the front to show the name of the player or other things like "dropbox 64."
3. The most annoying thing in multiplayer is to either have creepers blow up what your building or have a stupid griefer come and ruin you. There should be a way to prevent this, without the modded mode.
4. Bookcases, music players, torches, and glass are really the only forms of decoration. There should be couches, chairs, lamps, drawers, and even placeable books that the player can make.
5. On the books part, if the books are placeable, then they need to be able to be opened and have writing in them. Probably as much writing as a sign would, but in half so it looks like a book.
6. Horses. They already have rideable pigs, so why not rideable horses. 
7. There needs to be more animals. Horses, birds, penguins, bears, etc.
8. Underwater is really boring. Vegetation should be down there along with sea animals.
9. Along with the underwater, players should be able to craft diving suits out of glass and iron or some other metal. The first diving suits (SCUBA) looked like something in minecraft. These suits would allow them to stay underwater much longer.
10. If Notch added sea animals to the deep water, players need to be able to build spears. That would be so cool to go spear fishing in a diving suit in Minecraft.
11. Minecraft needs season cycling. The Quandry texture pack already has this, but it is such an integral part of the game that it should be included as default.
12. NPCs? Maybe? Every now and then, or even rarely, it would be fun to discover a camp of players in the game. To raid the place and take all their stuff would be even funner. 
13. If NPCs, imagine the thought of discovering ruins of an old civilization. Or digging through a cave deep underground and finding artifacts that can be used for more advanced building. 
14. Rare tools and weapons should be buried in places on the map. Digging out the item gives it too you, like an iron pickaxe. Maybe while digging, there could be rare things that you can't build like diamond fishing poles or skeletons.
15. Skeletons of prehistoric Minecraft animals. Slowly digging up the parts until it is complete, and then once it is, the animal would be added to the biome.
16. Buildable wall clocks, thermometers, compasses for inside a house or cave. Placeable. 
17. More materials! Why can't we make tents out of pig skins or clothes from feathers? 
18. More plants to grow in farms. So far, the only thing I've been able to grow is wheat. Rice? Corn? Potatoes? 
19. If a player want to build a huge farm, it is nearly impossible because animals spawn and destroy everything (I HATE CHICKENS). Players should be able to build scarecrows to keep them from spawning inside the farm.
20. This list is getting big. I have a lot more to add, it would get to 50, but these are the ones that came to my head first. For 20, players should find other players that are NPCs. NPCs would make the game so much more different. They could be your friend and give gifts, or your worst enemy, turning day into a zombie ridden night. If there are NPCs, there should be a larger list of weapons and armor. Raiding a castle in pigskin with a wooden sword will get you nowhere. 

Please comment on the list, give suggestions, and basically make Notch notice this list.