• Character can multiclass 4 classes.
• Ability Bonus cap increased to +18.
• Damage Resistance from items stacks
• Damage Resistance no longer exists for elemental damage, only physical. Feats granting elemental DR are now granting % immunity.
• Damage bonus of same type on a weapon now stacks.
• Weapon buffing spells do not stack, only 1 can be active on same weapon at the same time.
• 1 is not an automatic failure on saving throws anymore.
• Regeneration from items is per 10 seconds now. Regeneration from other sources (feats, spells) is unchanged, thus still per 6 seconds.
• Items with AC Dodge bonus (as we have more than boots) doesn't stack, highest applies. Dodge AC from other sources stacks normally.

• All spellcasters are now spontaneous and select spells known during level up.
• Spellcasters uses spell points to cast spells. The more spellpoints, the more spells they can cast.
• There are three types of spell points. Intelligence based spellcasters are using Arcana, Charisma based spellcasters are using Power and Wisdom based spellcasters are using Mana. If you combine two spellcasting classes of same casting ability, you can share the spellpoints.
• Casting a spell costs [spell level*2-1] spell points. Ie. 1st level spells costs 1 spellpoint, 2nd level spells costs 3 spellpoints etc.
• Number of uses will automatically recalculate anytime you cast any spell, rest, regain spellpoint etc. Additionally, a spellpoint bar will be shown next to the hitpoints bar left from the character portrait, that will visually show your spellpoint reserve.
• Items exists that grants bonus to specific spellpoints ie. Mana Bonus +20.
• Base spellpoints differs class to class, having high ability modifier also increases spellpoints of specific type.

This means that not only are all casters are spontaneous casters, all classes can now substitute “higher spell slot” for “lower spell slot”. So if you have 100 mana, you can cast 100x 1st level spell if you want, or 14x 4th level spell.

More information here: Spell Points :: d20srd.org