Gritty

Certain mobs are more aware than they used to be and can track you by noise and blood scent! Watch out!

There are seasons and certain crops will only grow during certain seasons.

Food now rots away after several days. Hope you get a freezer before Winter, because starvation is very painful.

There is a bunch of new food.

Stone works much differently now. If it is not exposed to air it will harden, making strip mining effectively impossible, find caves/mineshafts to get ores.

There are new, powerful undead enemies that can drop some rare goodies.

Fish are their own entities now, and can be picked up and put into a tank where they won't despawn. They make great decorations.

You can only see your coordinates with a compass or an Antique Atlas now.

Backpacks! Gain Extra Inventory Space.

Until you set a spawnpoint, you spawn randomly.

Lots of new Structures to explore.

New biomes to find.

Powerful Items that will help you not die to mobs.

Water is only now only  infinite in certain biomes.

Quark (a mod by Vazkii)

Enhanced Surroundings and Visuals!

Nature's Compass!

And much much more... but I will leave that for you to discover.