Starting off Minecraft making it much more complicated than it needs to be, with new animals, new plants, new crops and food recipes, new biomes, mechanics, and a new exploration experience both above and below the sea level overall.
The new things you will find in your world are all but not limited to:
Sea creatures, both real and mythical, from Abyssal Depths and Upgrade Aquatic
Majestic beasts only heard of in legend, from Ice and Fire: Dragons and WyrmRoost
A huge variety of new animals, each with their own use for the player all while keeping the vanilla feel from Alex's Mobs
Improvement on aquatic life with Fins and Tails and Aquaculture 2
Tons of new biomes, trees and plants from Biomes O' Plenty and Terra Incognita
A whole new cooking experience with new crops and tons of new recipes, brought in by Farmer's Delight and several of its addons and compats, and the Neapolitan mod
New items and creatures in the Nether, (since we've all gotten pretty used to the vanilla Nether) from Shret Nether
And a huge upgrade to the environment of Minecraft overall!