UI
Inventory System:
The bag system will be pretty much like WoW, the items will now only take one slot each and some items will be stackable. The bag contains 30 slots. You will also be able to equip more bags so you can carry many more items then befor.
They also improved the sytem so when you drag over an item, lets say a weapon you will see your equiped weapon at the side of it so you can compare them easier. That was also taken from WoW.
Now insted of only showing the weapons attack speed and damage you will see the dps ( damage per second ) so its easier too see what weapon that do the highest damage. And again, its taken from WoW.
The equipment slots is: Head, Shoulders, Neck, Hands, Chest, Leggings, Waist, Feet, Finger R/L, Weapon 1/2.
Mini-Map:
The mini-map is on the top right of your screen, it shows name of the area that you are in right above it.
Buffs:
The buffs you have is shown at the bottom left side of the screen.
Mana Bar:
In Diablo 3 all classes will not have a mana bar. Melee classes like Barbarian will insted have a so calld "Frenzy Bar". I think it will work pretty much like the Rage or Energy bar in WoW.
Combat Log:
In Diablo 3, when you attack an enemy, the damage you do will appear above the enemies head. This is a great change from Diablo 2 where you could not see the damage you do on an enemy when fighting fight.
Rarity:
Shows you what kind of Npc/Enemie you are targeting
Quests
Diablo 3 quests will be different from Diablo 2, there will be many more quests and they should be more fun. The quests will also have a better tie to the game then befor.
Blizzard: "We will continue to insert as many quests as we like, until we feel that we’ve got a good amount and there aren’t too many. We want to concentrate on quests that are really fun and contribute to the game. If I have to come up with a number, I’d say there’s probably twice as many quests, but I wouldn’t nail it down like that. It could still be more or less at this point."
Health System
It is known that the healthsystem will change in Diablo 3, and it will not be like in Diablo 3, where its all about who could carry/drink most pots.
Insted enemies will drop healing orbs.
Blizzard: "Health orbs heal a percentage of life, so the more health the character has, the stronger the effect of the orb. We’ve implemented a system in order to accomplish this. At the moment, we intend healing orbs to heal 15% of health. So if you have 1,000 health, the orb regenerates 150 health, and if you have 100 health, the orb regenerates 15. On the other hand, we can have it so certain monsters, like bosses, drop several orbs, or drop orbs as its own health dwindles. So for example, there can be a chance for an orb to drop every time his health drops by 5%, or force orb drops at certain points in the fight. There are also opportunities for orbs to influence objects or skills. Orbs don’t fill life instantly, they regenerate life."
Achievements
Diablo 3 will have Blizzards new Achievement system when it will be released, where you will be able to reach different goals such as, destroyed bosses, killed monsters and explored new areas.
Leveling system
You gain levels by killing monsters and gain experiences. New Levels unlock new features like new abilites, spells, talents and items.
Blizzard is planing to make 100 levels in Diablo 3 when there was 99 levels in Diablo 2.
Skill Rune System
The skill rune system is a new system created for Diablo 3. You will be able to attach a rune to skills. This will change the ability of the skill you attach it to. You will be able to change a normal spell like “teleport” to a spell that does damage byadding a Striking rune to it.
Known Runes:
Multistrike Rune
Electrocute
Striking rune
Hardcore Mode
If you die in hardcore mode your character will die forever and you cannot continue to play with it. Hardcore mode is taken from diablo 2.
Waypoints
In Diablo 3 you will be able to teleport to existing players from a so calld "Waypoint" that is in the towns. This will make it easier to reach them.
World Map: