Could be enemies, biomes, blocks, etc. I would like to see volcanoes, glaciers, a sky dimension, dragon lairs, bats, and a giant. Also, I wish the npc's had a purpose. They are just there, and do nothing.

Birds.

Well, something that flies. I dunno, something really dead about a world where the only thing in the sky is clouds.

I wish there was a spherical (cube?) world option. There's something nice about being able to walk in one direction for practically forever, but there's also something compelling about having a limited world. And going in one direction and, after a long time, realizing you're back where you began.

Quests and leveling...like they said it would have ages ago...

You know they have leveling now, and its required for enchanting tools and weapons. No quests though.

Stained Glass would be nice. Imagine coloured light filtering through a huge stained glass window in a massive, majestic cathedral. Awesome.

For some reason this seems like a dick move but...

PRetty much everything that is in terraria would spice minecraft up a whole lot.

more blocks. Each current block should have a stairs and half block version. With the exception of maybe ores, sand, and gravel.

The need to not rent servers every time you want to build a world with friends.

Rare monsters. Something like what Bigfoot is or the Loch Ness monster (You know, cryptids). Basically, they're in the world but they take a lot of exploring to find. I'm not looking for rare materials to drop from them, I just want a surprise when I explore the world.

I'd like more method to the ore placement. Something that will allow me to systematically go after specific ore. I want to be able to have an 'Iron Mine' not just a 'Mine'!

@Ubersmake said:

I wish there was a spherical (cube?) world option. There's something nice about being able to walk in one direction for practically forever, but there's also something compelling about having a limited world. And going in one direction and, after a long time, realizing you're back where you began.

I think there might be a mod out there that does that.

robotic sentries.