I've compiled a bunch of tips I picked up from playing the beta and following the game to help ease the transition from D2 to D3.
First thing you'll want to do is go into "Options > Gamplay" and activate "Elective Mode". Doing so gives you the freedom to assign skills as you wish without being constrained by arbitrary skill categories. Veteran Diablo players would no doubt find this frustrating, and frankly I wouldn't be surprised to see this changed before release since it's a huge point of contention among the beta community.
Set builds are a thing of the past. Players no longer gain attribute and skill points as they level. They gain pre-defined attribute points automatically and unlock skills/runes progressively up to the level cap of 60.
Players can have at most 6 active skills (each of which can have a single rune augmentation among 5 choices), along with at most 3 passive skills. As players level they gain the ability to equip extra active and passive skills.
Skills/runes can be swapped out with complete freedom, and doing so invokes a 30-second cooldown before that skill can be used or swapped out. Swapping skills in town does not trigger the cooldown. If you're swapping out multiple skills it's recommended to TP back to town and do so without cooldown to minimize downtime.
By default skill descriptions are simplified. Hold down CTRL to view more advanced descriptions with actual data. Choosing the gameplay option "Show Advanced Tooltips" will always show these instead.
Attributes in D3 are Strength, Dexterity, Intelligence and Vitality.
Strength: +x to Armor, +x to Barbarian's damage.
Dexterity: +x% to Dodge chance,+x to Demon Hunter's damage, +x to Monk's damage.
Intelligence: +0.1 per point to Resistances, +x to Witch Doctor's damage, +x to Wizard's damage.
Vitality: +x to Life
Each class has unique resource systems.
Barbarian - Fury: Attacking monsters (and inanimate objects) as well as receiving damage generates fury. Certain skills contribute to fury gain, while other skills expel it.
Demon Hunter - Hatred/Discipline: Hatred fuels her direct attack skills (bow skills, for example) and generates quickly. Discipline fuels some of her defensive and trap oriented skills and generates most slowly. Some skills function as hatred generators.
Witch Doctor - Mana: This is the only class to use mana which functions exactly as it did in D2.
Wizard - Arcane Energy: Similar to mana, only arcane energy generates more quickly. Signature skills (her basic attacks) do not cost energy.
Town portals are no longer scrolls but instead has been worked into the UI to be used whenever you please. You eventually earn this as a quest reward.
Identify scrolls are gone. Occasionally you'll receive unidentified items which you merely need to right-click to identify.
In parties there will be a banner for each of your allies in town. Clicking the banner will instantly teleport you to that ally.
Banners can be customized from inside B.NET and are constructed of various pieces you've unlocked through achievements. They're purely cosmetic.
Monster hitboxes are exaggerated in this game and can lead some some frustrating and buggy movement, especially when using a ranged attack on your LMB. There's a separate keybinding for "Move" which will instruct the player to follow your cursor when held down. I mapped this to my mouse thumb button for easy use. Train yourself to use this amidst packs of monsters for precise and stratetic movement.
Don't sell any blue/yellow/orange items early on in the game. When you unlock your blacksmith these can be salvaged for crafting materials which are precious early on. Crafting mats can be used to craft equipment and weapons. White items cannot be salvaged.
SHIFT-clicking continues to anchor your character in place as they attack.
There's a limit of 10 characters per account.
Any gold accrued is account-wide and stored in a single bank to be withdrawn from by all characters. Likewise, there's a single stash which you can store items in to pass off to other characters. There's a separate stash/bank for hardcore vs. softcore characters in your account.
It's entirely possible to make money from playing this game via the in-game auction house. Hold onto items you think are worthwhile for selling, unless you prefer to sell or salvage them. You have the option to sell/buy items on the AH using either in-game gold or real life money.
Your shared stash initially offers 14 blocks, and can be expanded by an additional 14 blocks for 10,000 gold. That gold is precious early on, so a trick to conserve space is to offload items from your stash onto other characters in your account until you can afford to expand your stash.
Artisans (Blacksmith and Jeweler) can be upgraded with gold and later on pages of training. Doing so allows you to craft better items and jewels. Artisan progress is account-wide and affects all your characters.
Look for the occasional book in towns and outside. Clicking a book will reveal a bit of lore and give you a slight EXP boost.
Keep a watchful eye for environmental traps like hanging chandaliers or loose walls. These do not harm the player but can be dropped on monsters for kill bonuses.
Players can earn EXP boosts by chaining monster kills, inanimate object destruction, and environmental kills.
Health globes occasionally drop from monster kills. These are floating red orbs. Picking these up will instantly heal you and all your allies/pets. Some passive abilities modify health globes to also affect class resources or other stats.
Mana and rejuvination potions are gone. Health potions still exists (Default key is Q), but there's a long cooldown on when you can use them again so use them sparingly.
You can no longer run around with the map overlay expanded. Clicking with the map expanded will instead pan the map rather than direct your player.
Player death incurs a durability hit on your equipment (you don't lose gold or experience). Dead players can be temporarily revived by allies before they're returned to the last checkpoint.
Num keys evoke player emotes. #6 is the classic "Time to Die".
Magic find (MF), +EXP, and +Gold affixes are averaged among all players in co-op.
Loot is individualized to players, meaning whatever you see dropped is yours and yours alone. If you drop an item from your inventory on the ground all players can see it.
Keep a watchful eye for the elusive Treasure Goblin. He's a glowing monster that rarely appears, and if you kill him he'll drop a lot of loot. Upon discovery they'll often run away leaving a trail of gold, and attempt to warp out off screen. Kill them before they get the chance to do so.
Walking over gold piles automatically picks it up.
Turn the gamma up slightly in video options. It improves graphical clarity and makes the game look better.
Items sold at a vendor can be bought back for the same price.
Activating a shrine shares the buff with all party members.
There are 3 followers players can hire throughout the game. Followers are reserved for solo play only and cannot be used during co-op.
Pressing "G" will drop your banner on the field, allowing players who click on it to teleport to you.
Beyond normal, nightmare, and hell...a fourth difficulty has been added called inferno. You must be level 60 to play here.
Strength and dex are no longer a requirement for gear and weapons. The only pre-requisite is character level (clvl). Some items are class-specific, but beyond that all classes can don the same equipment. The same equipment will look differently on the various classes.
When you die you become a tombstone and can be resurrected by your allies for a brief period of time.
Death during a co-op boss fight will lock you out completely with no way to continue. If you die make sure your ally resurrects you.
+ Magic Find, + Gold Find and + XP will be equal for each character in the group and depending on the sum of bonuses for all players. For example, if in a party party group the total of their stats are 50% MF, +20% GF and 10% XP, each of these players will have a 12.5% MF, 5% and 2.5% GF XP.