Here is a short description of some of the aspects of 3E AD&D as they are (most likely) used in Icewind Dale II (as found at the IWDII forum).

How does Multiclass work?
Pros:

• You can, at any point in your adventuring career, opt to take almost any other class when you level up.
• Humans can multiclass.
• No split multiclass xp.
• No different levelling rates except for racial xp penalties and the exception listed below.
• Any race can be any class.

Cons:

• If your classes are 2 or more levels apart and one of them is not the class favoured by your race (Humans and half-elves favour any, and the favoured class is their highest class.), you have a 20% experience penalty per class not in range.
  (Examples: Dwarf (favoured class fighter) with 6 levels of fighter and 3 of thief, no penalty. 6 fighter, 3 thief, 1 wizard, 20% penalty. 6 fighter, 3 thief, 2 wizard, no penalty. 6 fighter, 3 thief, 1 wizard, 1 cleric, 40% penalty. Favoured class is always out of the equation.)

How do you level up?
If you have a level each in fighter, mage and rogue (no 'thief' in 3e), instead of that experience for the goblin being split between the three experience totals, 7 to fighter, 7 to mage, 6 to rouge, all 20 points go towards your next level, you have only one experience point total.

When you reach your next level, you can choose to become a 2nd level fighter/1st level mage/1st level rogue, or 1st F/2nd M/1st R, or 1st F/1st M/2nd R, or even opt for another class altogether, 1st F/1st M/1st R/1st Cleric.

You can't start the game as a F/M/R. You start as any class, say fighter at your first level, you add a level of mage for your second, at the third you go for rogue. Or you can be a straight 3rd level fighter, or 3rd level mage, or 3rd level rouge, or 3rd level any other class, or 2 levels in one class and one in another.

You add the levels in each of your classes to reach your character level.

What about feats?
Feats will be implemented in Icewind Dale 2.

Feats are unique abilities, one obtained at 1st level and one at every 3rd level thereafter (3rd, 6th, 9th, 12th etc.)

Feats include being able to create magic items/potions/scrolls etc, being able to wear heavier armour types or be proficient in weapon types not normally open to your class, special combat maneouvres such as dodging and deflecting arrows, trick shots with ranged weapons, better reflexes, better saves, special attacks such as cleave and whirlwind, abilities to use weapons better such as weapon expertise, weapon focus, weapon finesse, two-weapon fighting, ambidexterity.

All sorts of things.

Is there still dual classing?
Not per se. All classes are consecutive rather than concurrent. You can have a class of fighter, level up and choose a class of rogue, level up and choose a class of something else. There's an xp penalty for too many classes at too seperate a level, except for a race's favoured class, which for humans is 'any'.

Can all classes multi class (eg.Paladins, Sorcs?)

Yes, certain classes though, like Paladins and Monks have special rules.

J.E.Sawyers suggestion for Paladins and Monks:

Paladin of Ilmater - Freely multi- to cleric and back.
Paladin of Helm - Freely multi- to fighter and back.
Paladin of Mystra - Freely multi- to wizard and back (yes, in fact, Mystra does have paladins, let's not even bring up the whole "only LG deities may have paladins" argument again)

Broken Ones (Lawful Good Ilmateri monks) - Freely multi- to cleric and back.
Old Order (Lawful Neutral monks) - Freely multi- to rogue and back.
Dark Moon (Lawful Evil monks) - Freely multi- to sorcerer and back.

It's slightly different than what is in the FRCS hardcover, but it's pretty close.

Normally, if a character with Paladin or Monk levels takes a level in another class, s/he cannot take anymore Paladin or Monk levels.

Are race restrictions gone?
Yes, anyone can be any class, certain races have favored classes though, as explained above.

What about Alignments and Alignment restrictions?

Alignments, the same from 2e. Some class/alignment restrictions:
Barbarians, nonlawful.
Bards, nonlawful.
Clerics, within one step of their deity.
Druid, at least partly neutral.
Monks, lawful.
Paladins, Lawful Good only.

in standard 3E, a cleric cannot be True Neutral
unless his/her god is True Neutral.

What happens if I change alignment?
Bard: Retains abilities, cannot take more bard levels.
Monk: Retains abilities, cannot take more monk levels.
Barbarian: Retains abilities expect rage, cannot take more barbarian levels.
Cleric: Loses all abilities, cannot take more levels, might be able to atone (per spell).
Paladin: Loses all abilities, cannot take more levels, might be able to atone (per spell).
Druids: Loses all abilities, cannot take more levels, might be able to atone (per spell).

NOTE: Abilities doesn't include Bab, saves, HP, feats or skills.

Some Favored classes
Humans: any, always highest level class
Half-Elves: any, always highest level class

Aasimar: Paladin

Tiefling: Rogue

Elves (standard): Wizard
- Wood: Ranger
- Drow (female): Cleric
- Drow (male): Wizard

Dwarves (standard): Fighter
- Gold: Fighter
- Gray: Fighter

Halflings (standard): Rogue

Half-Orcs: Barbarian

Gnome (standard): Illusionist
- Deep: Illusionist

Official 3E Rules
NOTE #1: official 3E rules. Don't know how BIS is going to handle
them.

In IWD2, ability scores are initially generated using a point-buy system. All statistics start at 10 and the player has 16 points to distribute. Racial modifiers are applied to the initial scores, do not subtract from the point pool, and modify the minimum and maximum value of ability scores. E.g., an elf would start with a 10 Dexterity, 6 Constitution, and could get as high as a 20 Dexterity and as low as a 1 Constitution. Every four levels, a character receives a point to place in the statistic of your choice.

When possible, ability scores follow 3E bonus scaling. In almost all cases, the character gains a higher bonus for every 2 points above 10, a lower penalty for every 2 points under 11. E.g., a character with a 14 Strength would have +2 attack bonus, +2 to damage. A character with a 9 Dexterity would have -1 attack bonus (missile), -1 to armor class, and -1 to reflex saving throws.

Here are the things that ability scores modify:

Strength: Melee attack bonus, melee damage bonus, carry weight, bash doors
Dexterity: Missile attack bonus, armor class, reflex saving throws.
Constitution: Bonus hit points (not class-restricted), fortitude saving throws, caster concentration checks.
Intelligence: Starting weapon proficiencies, chance to learn spells, wizard bonus spells, dialogue options.
Wisdom: Will saving throws, priest, paladin, druid, ranger bonus spells.
Charisma: Store prices, dialogue options, bard and sorcerer bonus spells, turning undead, paladin saving throws.

Bonuses granted to ability scores do not stack. The highest available bonus is applied. E.g., a character is wearing a ring of strength that grants a +1 bonus. The party sorcerer casts a strength spell on the character, granting a +3 bonus. The target will only use the +3 bonus.

NOTE: ALWAYS round down

Bab's (Base attack Bonus, THAC0 = 20 - BAB):
Fighter classes: Bab=level
(Fighter,Paladin,Ranger,Barbarian)

Cleric/Rogue classes: Bab= 0.75 * level
(Cleric,Druid,Monk,Bard,Rogue)

Wizards classes Bab = 0.5 * level
(Sorcerer,Wizard)

Bab's for multiclasses add up.

Additional attack at +1 for each total +6, ie +8/+3 or +15/+10/+5.

Saves:
Each creature has 3 saves, Fortitude (constitution based), Reflex (dexterity) and Will (wisdom). The total save is the ability bonus + class based bonuses.

Each class gives bonuses for all 3 saves; all saves are either low or high for each class and follow a similar increasement rate.

Saves for multiclasses add up.
Bad saves= level / 3
Good saves= (level / 2) + 2

Good saves (the rest for each class are bad):
Fort:Barbarian,Fighter,Cleric,Monk,Druid,Paladin,R

anger
Ref:Bard,Rogue,Monk
Will:Bard,Monk,Cleric,Druid,Wizard,Sorcerer

Ability Modifiers:
Modifier= (Ability /2 ) - 5
ie, strength +4= (18/2)-5

Bonus spells:
None for spell level 0
Ability bonus X = 1 bonus spell for levels X,
+1 for each -4X

ie, stat bonus +7, 1 bonus spell for levels 1-7, 1 extra
bonus spell for levels 1-3 (7-4=3)

Bonus spells are based on abilities:

Wisdom: Cleric,Druid,Ranger,Paladin
Intelligence: Wizard
Charisma: Bard,Sorcerer

Requirements for spell levels:
To cast a spell of level X, you need to have the
relevant ability of at least 10+X.