Quite a few major changes, actually. A lot of new blocks (quartz, coloured clay, stained glass, coal, podzol, more), mobs (witches, baby zombies, baby pigmen, chicken jockeys), passive mobs (horses, bats, clownfish, puffer fish, salmon).
A whole bunch of new biomes, or variations of existing biomes, including new trees, flowers and sometimes blocks to populate them. You can make "enhanced" worlds that exaggerates the vertical scale and results in some crazy land generation.
A lot of changes to redstone and other game mechanics, such as fishing. Too much stuff to list. Take a look through the version history pages for more...
http://minecraft.gamepedia.com/Version_history#1.6