Having spent some time reviewing the changes, and a little time in game, as well as the fact me and my friend Evic will be kicking off our new cooperative lets play this evening, I can confidently say I am excited to get into the new version of the game, and some of the changes I have seen or read about are awesome!
My world preview video for the Coop Lets play world.
There is a new option on world creation to create a custom world. With this, you can use a selection of sliders and check boxes to decide what is in your world, and how much presence for certain things there will be. A fantastic way to create challenge maps, or just something that looks a little different.
In ocean biomes, there is also the chance of discovering an ocean monument - large underground structures containing rare materials and guarded by new mobs - the guardians. To say they sound challenging is an understatement. Physics changes mean that arrows wont work under water (beyond 2 blocks) so you need to get up close to them, and to do that likely means crafting lots of water breathing potions.
On the subject of mobs, there are a lot of mob changes and additions. There are passive mobs, such as bunnies, that can be killed for food and leather scraps that can be combined to make leather. Guardians, which guard the monuments, and Endermites, which spawn around nether activities, like Endermen teleporting, and are similar to Silverfish. Sheep now drop meat, bunnies drop meat and can be used with a large recipe to make Rabbit Stew that restores 10 food and 12 sustenance, making it the most potent food in the game. Additionally, the Wolf AI has changed, they now actively hunt skeletons, and scare them away. Give a dog a bone eh?
Villagers have had an overhaul. The trade system is more effective and there are more unlocked trades, and villagers also have a level of willingness that goes up as you trade with them, making them susceptible to breeding with other willing villagers if they get to a certain level. They also have professions, which will dictate their tradeable goods. Best keep them villagers alive ;-)
Enchanting levels have been changed, and enchanting also uses Lapis Lazuli. Finally, a use beyond blue dye for this most useless of blocks.
Some other changes include making certain items stackable, like doors, and also making wood items like doors, fences and the likes actually have colour to match the base material. This makes for much better customisation. Other items include armour stands where you can hang up armour or mob heads - which themselves are gathered from blowing up mobs with a charged creeper. THere are also a load of new blocks ranging from new types of stone, to slime blocks (trampolines!) and sponge blocks (hold water).
On top of this there have been a butt load of tweaks and bug fixes. If you have not had the chance yet, get in there and enjoy!! :D