Dungeons & Dragons (D&D), originally published by TSR, and currently published by Wizards of the Coast, has had five printed editions since first published in 1974, counting the original, unnumbered rules set as an edition. In 2008, Wizards of the Coast released 4th edition Dungeons & Dragons (technically the 5th edition). As is common with any new rules edition of a role playing game, 4th edition introduced fundamental changes to both the mechanics and story elements of Dungeons & Dragons.
For anyone familiar with previous editions, some elements of the newest edition will feel familiar and some elements will seem very unusual. Which elements of the game have changed depend primarily on which edition a person is familiar with. Of the previous editions, original D&D, 1st edition, and 2nd edition were rather similar to each other. 3rd edition D&D was a rather sharp departure from previous editions. The best way to examine the changes is to consider those changes in relation to either 2nd or 3rd edition.
2nd edition to 4th edition
For those who have not played D&D since 2nd edition, the mechanics are the biggest changes. The basic idea of six attributes ranging from 3-18 still exists as does the idea of rolling a d20 to determine the outcome of an attack. Most of the base classes and races still exist, though many new ones have been added. The game still uses a level based class system that mixes magic and swordplay to fight mythical creatures. And, the background story for these two editions is mostly the same, with the largest change showing up in the cosmology of the D&D plane structure.
But, while similarities are apparent, so are many differences. Difference in characters would be the most apparent to most players. Character creation has moved away from random dice rolling. A point allocation system is now the primary system of generating attribute values. Also character subsystems have been normalized. No longer is the magic system entirely different than the weapon combat system. Each character uses the same system of action resolution regardless of whether using might or magic. Also, in order to simplify the math involved in the game, higher numbers are always better than lower numbers, including for armor class (AC). Finally, saving throws as a defense mechanic have been removed and replaced with scaling defenses similar to AC.
Combat is where 2nd edition players will observe the most changes. The combat in 4th edition is highly miniature centric and essentially requires a square grid map to effectively run. Combat is based on a power system, where every character has a list of powers that can be used with varying frequency. Some powers are incredibly simple and can be used every round. The most basic power is called a basic attack and while it is described as a power, it is simply the act of attacking with a weapon, in nearly precisely the same way that attack was made in 2nd edition. More complicated character powers can allow a warrior to trip with an sword attack, allow a ranger to fire three arrows at once, allow a wizard to throw a fireball at a group of enemies, or allow a cleric to heal an ally while smiting an enemy. The nature of the powers are primarily dependent on the class of the character.
With the introduction of the power system in 4th edition, the idea of Vancian magic is mostly a memory in D&D. Vancian magic is the technical term for the system of magic that required spellcasters to memorize spells that was present in previous editions of D&D. Instead, powers may be used at-will, once per combat encounter, or once per day. This idea of a power system permeates 4th edition. Monsters use a nearly identical power system to player characters, with slight changes in how often monsters can use powers. Magic items are also written as powers and can generally be used either once per day or once per combat encounter.
3rd edition to 4th edition
As far as mechanics go, 3rd edition and 4th edition are very similar. The power system described in above is a departure from 3rd edition, but character creation, and combat mechanics are otherwise very similar. The skill and feats system introduced in 3rd edition both continued into 4th edition, though both systems have been streamlined. The ability to freely multiclass that was introduced in 3rd edition no longer exists in 4th edition and 3rd edition saving throw values have been changed to static defenses, while maintaining the feel of fortitude, reflex, and will saving throws. Finally, all classes have the same attack bonus progression and classes no longer gain multiple attacks for higher attack bonuses.
While the above may seem drastic, in actual play a 3rd edition player would feel very comfortable stepping into a 4th edition game, at least as far as mechanics go. The more notable changes are in background story and design approach of mechanics. The 3rd edition game set Greyhawk as the primary setting. Nearly all of the background story for the game detailed story elements specific to Greyhawk. This meant that 3rd edition had a very well fleshed out setting, but that it was specific to one D&D world setting. Many of "classic" story elements from previous editions were altered to match the setting. Many of the canon elements from 3rd edition no longer exist in 4th edition, since 4th edition took many of its story elements from 2nd edition and before. Most notable is that Vecna and githyanki play a much less prominent role in 4th edition and Orcus plays a much more prominent role.
The other notable change is in design philosophy. 3rd edition D&D was designed to be a reality simulator. This not true for 4th edition D&D. The current edition is designed to entertain primarily and simulate as a secondary function. This philosophy permeates the system and the publishing philosophy. At a micro level, this is most evident in monster design. Monster statistic boxes only include information necessary to run that monster for a single encounter. This streamlining of detail, makes the game easier to run for DMs.
At a macro level, this philosophy effects the content of published books. Every effort was made to make it entirely unnecessary for a player to own or read the Monster Manual or Dungeon Masters Guide, unless that player planned to run a game. All books are designated as a player resource or a DM resource and while the DM needs access to player resources, the reverse is not true. Furthermore, the goal was for a player to need no more than one or two books at the gaming table to run a specific character. In third edition, players consistently needed five or six books for all their character rules and DM and player material was often intermingled in the same books.
It is impossible to detail every change in 4th edition D&D in just a few paragraphs. Those changes fill three books totaling approximately 800 pages. Still, the previous few paragraphs should provide an introduction to players familiar with previous editions that will make it easier to understand the newest edition of D&D. For anyone trying to learn about the new edition, whether familiar with the previous editions or not, the same advice holds true. The best way to learn about 4th edition D&D is to read the 4th edition Players Handbook with an open mind and as few preconceptions as possible.