Dungeons and Dragons Third Edition (thing)

Characters

• The class "groups" are gone. Therefore, rangers and paladins are no longer types of fighters, druids are no longer a type of cleric, and bards are no longer a type of rogue.
• All characters now have a "character level" in addition to their class level(s); standard characters have a maximum character level of 20 (though optional rules allow for this mark to be surpassed).
• Monk and barbarian are back as standard character classes, with completely reworked rules and descriptions.
• The sorcerer is a new spell-casting class that does not need to memorize spells in advance.
• "Thief" is now "rogue."
• Half-orcs are again a standard race. They can never (or almost never) pass for human, even an ugly human.
• Halflings are short and skinny, and do not have hairy feet.
• Gnomes are slightly taller than halflings and are also skinny.
• 25 is no longer the maximum for stats. The modifiers due to stat tables have been combined into a single table.
• The non-weapon proficiencies list has been split into "skills" (such as use rope, disguise, or spellcraft) and "feats" (such as blind-fighting, ambidexterity, and two-weapon fighting style). Both lists have been extensively revised; many items have been merged or removed, and many new items have been added.
• Weapon proficiencies are now feats. Fighters, Rangers, and Paladins automatically have the necessary feats to use any "simple" or "martial" weapon (such as clubs, polearms, swords, or bows) they come across without penalty; they still must take feats for individual "exotic" weapons (such as shuriken, dwarven waraxe, or bastard sword)
• The six thief skills are now skills that any character can learn at any level.
• Dual-classing has been removed from the system; any character can multi-class.
• Multi-class characters add all their bonuses, abilities, and hit points instead of averaging them or using the best.
• All clerics use the same spell list. Each cleric also selects two of the deity's "domains" (generally similar to 2nd edition's spheres), and gains access to another spell per spell level from each domain. Unless it is also included in the cleric's normal spell list, only one domain spell can be prepared each day.
• Specialist wizards choose which school(s) they will not be able to access, from a list unique to each school.
• All characters use the same experience point table, as it works off of character level, not class level. (Thus, a 5th/2nd level fighter/mage who wants to gain a mage level must get enough xp to go from 7th to 8th level in order to gain that level.)

Combat

• Rounds of combat last six seconds.
• Initiative is rolled only at the beginning of a combat; the initial order of action is generally kept throughout the rest of the combat. After the first round, there is no more "first" or "last", only "next" in a repeated cycle of actions. There are ways to change where a character is in that order, however.
• There are only three saving throws for characters: Reflex (getting out of the way), Fortitude (withstanding massive physical damage), and Will (fighting off mental attacks).
• The rogue's "backstab" is now a "sneak attack," and can come from any direction. Instead of a damage multiplier, it does an extra 1d6 damage at 1st level, 2d6 at 3rd, 3d6 at 5th, etc.
• Critical hits in the form of damage multipliers are now part of the standard rules. A roll of natural 20 is not always required for a hit to be critical.
• Except for damage, almost everything is based on the roll of a d20. High is always good, low is always bad. In most cases, a natural 20 will always succeed and a natural 1 will always fail.
• THAC0 is gone; AC now starts at 10 and goes up. If the rolled number plus modifiers is equal or higher than the target's AC, it is a hit.
• Polearms have "reach" and are generally only useful if an opponent is ten feet away.
• Which direction a character is facing does not matter in combat. Instead of bonuses to hit for attacking from the side or rear, whenever two characters are on exactly opposite sides of the opponent they are fighting, they get a bonus to hit. If the opponents are thieves, they can "sneak attack" every round that they remain on exactly opposite sides of the opponent.
• Unarmed combat has been overhauled and streamlined.
• Characters are unconscious and dying if their hit points fall below 0, and die when their hit points reach -10.
• Each character in combat "threatens" an area for 5 feet in all directions. If an opponent attempts certain actions within this area, the attacker gets an attack of opportunity on that character.
• Use of miniatures of some sort (even just dice on a grid) is strongly recommended, due to the many instances where relative position of combatants and distance between combatants is vital to running a combat.

Spells

• All spells are now in a single alphabetical list, with a line in each describing which classes can access it and what spell level it is for each class.
• All spells have been extensively overhauled, with many added, dropped, renamed, altered beyond recognition, etc.
• Magic resistance is now called "spell resistance."
• The cleric spell list has been rearranged to be on a 9-level scale instead of a 7-level scale.
• All spell writeups now include a comment about how spell resistance applies to the effects of the spell.
• Spells generally have casting times of 1 action, 1 round, 1 minute, 1 hour, etc. Mages who cast a 1 action spell can also move up to their full movement for the round. Mages who cast a 1 round spell can move up to 5 feet that round.

Monsters

• Monsters do not all get d8 for hit dice; some may get d4, others may get d12. Bonuses to the final hit point total can now far exceed +3. All monster listings include the average number of hit points, so that "standard" examples of that creature can be easily created without rolling dice.
• There is now a save against undead energy drain to see whether it is temporary or permanent. Energy drain gives a character "negative levels," which apply a cumulative -1 to all rolls and will kill a character if they are equal or greater than the character's Character Level.
• Monster xp now varies depending on how much of a challenge the encounter is to the PCs.
• Special abilities are listed as Extraordinary, Spell-like, or Supernatural, to better judge how they interact with other abilities (such as spell resistance) and whether or not they can be disrupted in combat.
• Monsters now have STR, DEX, CON, INT, WIS, CHA, as well as saving throw bonuses, just like PCs.
• Monsters can gain levels in "character" classes, and their monster hit dice count as levels of "Monster."
• Dragons have been made significantly more dangerous and deadly than before.

Miscellaneous

• All PCs, NPCs, deities, and locations mentioned in the PH and DMG are taken from the world of Greyhawk.
• Far, far too many details to list here.

Changes courtesy of the rec.games.frp.dnd faq.