I'm not digging through all the other posts to quote people.
- Stacking of different slabs/planks together to make a block.
- Less lag, like everyone says. I like the fog of war loading that Cube World is doing. They drew inspiration from you, why not borrow a bit of inspiration from them. The fog allows for a progressive load unseen while still giving basic outlines so the player knows what is coming.
- Furniture that is not a mod so that Texture Packs don't have to account for every little mod.
- Other races. I like the mention of Dwarves I came across by adrianstameski. Maybe also Elves or at least some tree dwelling race for the Forest/Jungle Biomes. Swamp villages for the Swamp Biomes that will actually use boats to get around.
- Scythe tool. You have hay, hay bales, sugar cane, etc...why not a proper tool for harvesting it.
- Wider trees, especially for the suggested race addition. 3x3 and larger.
- Rotatable (spelling?) slabs to be able to create ceiling joists...or just give us planks that center mount like fences. Understandable if this is only in creative mode.
I know the lag thing for example is partially due to resolution and the capabilities of individual computers so that may never truly get fixed, i just relay liked the fog idea