Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Bloodletting - Kill 25 elite (champion, rare, or unique) enemies in cooperative games.
Slaughterhouse - Kill 50 elite (champion, rare, or unique) enemies in cooperative games.
Due Decimation - Kill 100 elite (champion, rare, or unique) enemies in cooperative games.
Massacre of the Damned - Kill 500 elite (champion, rare, or unique) enemies in cooperative games.
The Culling of Hell - Kill 1,000 elite (champion, rare, or unique) enemies in cooperative games.
Heroes of the Ages - Join a cooperative game.
Gotta Catch Them All - Play cooperative games with the following hero classes.
The Great Revival - Resurrect a player of each class in cooperative games.
Life is Beautiful - Heal other players for 500,000 Life by picking up health globes in cooperative games.
Follow Me Around - Teleport to another player by clicking on his or her banner.
Warriors of Sanctuary - Join a 4-player cooperative game where everyone is a different class.
Heroes of Circumstance - Join a public cooperative game.
Not Dead Yet - Resurrect another player in a cooperative game.
Breath of Life - Resurrect other players 25 times in cooperative games.
Savior of the Fallen - Resurrect other players 50 times in cooperative games.
All - Campaign
Grand Beastmaster of Sanctuary - Complete the bestiary achievements listed below.
A World of Pain - Complete the exploration achievements listed below.
Grand Lorekeeper of Sanctuary - Complete the lore book achievements listed below.
All - General
Scavenger - Pick up 10,000 gold
Spare Change - Pick up 50,000 gold
Moneybags - Pick up 100,000 gold
Deep Pockets - Pick up 500,000 gold
Staying Gold - Pick up 1,000,000 gold
All That Glitters - Pick up 5,000,000 gold
Spoils of War - Pick up 10,000,000 gold
The One Percent - Pick up 100,000,000 gold
Armed to the Teeth - Equip all inventory slots with items that have a minimum required level of 25 or higher
Prepared for Battle - Equip all inventory slots with items that have a minimum required level of 60 or higher
The Comfort of Strangers - Recruit the Templar, Scoundrel, and Enchantress
Breaking Not So Bad - Use a merchant to repair an item
All - General Exploration
A Guiding Light - Use the Templar as your follower.
Me, Myself & Eirena - Use the Enchantress as your follower.
Stolen Moments - Use the Scoundrel as your follower.
Friends with Benefits - Fully equip one of your followers.
Rarely There - Equip a rare (yellow) item.
Legends of the Brawl - Equip a legendary (orange) item.
Chestmaster 2000 - Open 2,000 chests.
Born to Dye - Dye an item.
All - General Conversations
The Art of Conversation - Complete the main character, follower, and artisan conversation achievements listed below.
Now I've Heard Everything - Complete the main character conversation achievements listed below.
Dirty Little Secrets - Complete the follower conversation achievements listed below.
Talkis Cheap, Friends Are Priceless - Complete the artisan conversation achievements listed below.
For the Order - Listen to all of the Templar's conversations.
To Catch a Thief - Listen to all of the Scoundrel's conversations.
Secret of the Ages - Listen to all of the Enchantress's conversations.
Hassling Haedrig - Listen to all of the Blacksmith's conversations.
Everybody Loves Shen - Listen to all of the Jeweler's conversations.
Stay Awhile and Listen - Listen to all of Deckard Cain's conversations.
More than Stories - Listen to all of Leah's conversations.
It's Just Us - Listen to all of Tyrael's conversations.
Bewitched - Listen to all of Adria's conversations.
Sales and Tales - Listen to all of the town merchants' conversations.
Attacks are easier now as You can hold down Your mouse button while you have an enemy selected to repeatedly attack it. You don't have to click for each attack as before.
If Yold the CTRL button while mousing over an item on the ground You will reveal its stats. This in turn will provide you with a comparison between the item on the ground and the equivalent item you have equipped.
Dual wielding weapons gives you a 15% attack speed bonus.
If You hit the ALT key You'll be able to view the names of all the items on the ground for 10 seconds. Duration can be changed in the options.
There is a "Force Move" (titled as "Move") command that is unbound by default. If You bind it to one of your additional mouse buttons it can be usefull.
Lore books provide experience points.
Treasure goblins will disappear if you don't kill them fast enough. They tend to drop a lot of gold and magical items.
Press the "G" key to plant your banner on the ground.
Press the "Z" key to get up close and personal with your character
Press the "X" key to view the health bars of all the monsters on screen.
The Numpad buttons on your keyboard allows your character to say pre-recorded lines that are both audible and appear in the chat box.
To unlock Hardcore mode You'll have to get a character to level 10.
Whn playing with a ranged character class like Witch Doctor, Wizard, or Demon Hunter if You hold down the mouse button and hit the SHIFT key they'll stop and attack at the direction You're pointing in.
To inspect other players and look at what they're wearing just hit right-click for their character portrait.
Playing with a friends!? You might considet tactics to have one player as the party's blacksmith and the others to focus on jewel crafting.
Crafting artisans are account-wide. This means that everything you do with with your artisans will be carried over to all your characters.
Leveling up your artisans and craftsmen is important for salvaging items, socketing items, and crafting powerful gear.
Looking for the best items in the game!? It is easier with exploration and killing elite packs of monsters than bosses.
There is an XP penalty for killing monsters too different from your current character level. If the level difference is less than 5, there is no penalty. If the difference is between 5-10, there is a small penalty. If the difference is over 10, you'll only receive 5% XP for killing the monster. You will gain bonus XP for killing monsters up to 3 levels above your level.
Finally, to earn money You'll have to go to the Real Money Auction House (RMAH) and sell your items there. In the RMAH, you have the option to earn "Battle Bucks" or cash it out through PayPal. "Battle Bucks" reside in your Battle.net account and cannot be cashed out. You can use "Battle Bucks" to buy items on the RMAH without spending real money.
Save Your Experience Gear
Experience gear can cut your play time by as much as 20%. But you need to keep upgrading your gear or else the bonus experience won't be enough for your current level. Having a bunch of sets of experience gear every few levels will help. Just so you know, you don't really need full sets, just a few pieces to make a difference on your next character.
Also, if you are running out of space and don't want to upgrade your shared stash, the best way to keep them is to create a few extra characters and pass the gear to them using the shared stash.
This will also save you money, but eventually, you should upgrade your shared stash to avoid wasting time moving items between characters.