2nd Edition AD&D presents a number of optional rules DMs can implement in their campaigns if they wish. Later books in the line, the Players' Option books and the revised core rulebooks, offered alternate rules to customize games according to preference. Some of these rules were incorporated into 3rd Edition, yet in 2nd, none are necessary to play the game.
AD&D 2nd Edition cleans up and clarifies a lot of the rules, but the core mechanics are the same. Adventure modules, monsters, etc. written for 1st Edition can be used with little or no revision.
TSR published many fully-developed and well-received game worlds for AD&D 2nd Edition, spanning a variety of fantasy themes. Examples include Dark Sun, Hollow World, Planescape and Spelljammer.
2E causes players to re-think their decisions before engaging in risky behavior. Death can happen at any stage, and there won't be a chain of saving throws to stave off the reaper: You reach 0 HP, you're dead! Much of the focus, then, reverts to problem solving and role playing.
Due to concerns over public perception, for 2nd Edition TSR chose to eliminate mature themes and explicitly "evil" player options; remove references to angels, demons and devils; and otherwise smooth over potentially offensive content in the core game. This deliberate sanitization is easily reversed in personal play.
Basic D&D is 6 ability scores, a few skills, a character's saving throws, some equipment and, if a spellcaster, some spells; and that's it. This makes the game more about what's happening in the game rather than an exercise in firing off various powers on your character sheet.
As with AD&D 1st Edition, Basic/Expert D&D is one of the favored editions of the Old School Renaissance community, who regularly offer wildly creative material for it and have published several retro-clone editions, all of which are currently available online either for free or for low prices. OSR blogs on Blogspot and Google+ communities serve as sources of considerable materials that can be used without modification for this edition, and some of these materials break far away from the "standard vanilla fantasy" flavor of Wizards of the Coast's publications.
Warriors with weapon mastery and wizards with spells at higher levels feel more on par with each other than in other editions, where the wizards easily overshadow the warriors of the party. Magic is the best in other editions whereas in BECMI they had a place in the party and it was usually in the face of some monster.
Lamentations of the Flame Princess is a horror/fantasy game (not for kids). Fully compatible with Basic D&D material, it features a few non-breaking adjustments such as a vastly simplified encumbrance system.
5th Edition strikes a balance in keeping the rules simple in some areas, confusing and poorly thought out in others, while limiting the creativity of players and any real choice.
Wizards of the Coast provides free PDFs containing the complete core mechanical rules of 5th Edition D&D (combat, exploration, spellcasting, etc.) and a selection of iconic character options, monsters, magic items, etc. drawn from the full game. Though short on variety, the basic rules are almost playable and accurate to the full game.
Of course far more is free in older editions as well as cheaper, 5e is by far the most expensive edition, and it is almost as bad as 4th edition.
Overuse of concentration mechanic, no racial stat modifiers and hardly any differences at all between races and classes, compared to previous editions.
Few core aspects of 5th Edition exist solely to encourage thinking in-character, including mechanical rewards for good roleplay and detailed character backgrounds with lasting effects in the game.
Bounded Accuracy is a core design ideal of 5th Edition. In short, it means the numbers don't get too big; this lets weak challenges remain relevant for longer, and strong ones become survivable sooner. In practice, it also places more focus on a character's inherent stats and less their class or on magical equipment than has been the case in recent editions, which is a another major downside to 5e.
As with other editions of the game, much of the content is released under the Open Game License (OGL) allowing custom modification, extension and alteration of the game.
Only justification for the system is to let critical role bring more people into the hobby. Once players try Warhammer, Vampire or AD&D 2e, they will have no need to return to 5e.
WoTC has been super lazy since 3,5 and even that had only a third of the material available that TSR published for AD&D 2e.
Is is sad that WoTC after 7 years has not supported 5e or other versions better.
This is a great reason to look into 2e or 3,5e systems instead of 5e.
This is the most recent edition of the game and is the only one for which official content is being produced. Do not expect to see new official adventures or splatbooks published for any of the older editions of D&D.
Later editions were, to varying degrees, designed for market appeal; but the original AD&D is little more than one man sharing notes on his personal vision. It's tailored to his thematic tastes, and very often as much care is put into explaining the reasoning behind a rule as into defining the rule itself.
The Old School Renaissance community has a significant online presence on Blogspot and Google+, and features some of the most creative minds in fantasy gaming, all offering their ideas to the gaming audience for free or for generally low prices on PDF storefronts. As they aren't bound by Wizards of the Coast's need to cater to a mass market, they aren't obligated to present only a single "flavor" of fantasy. People playing 1st Edition (or one of its many available retroclones) will have no lack of material and inspiration to draw from.
With so many options available to the player, and the almost unlimited ability to combine them, certain "character builds" are patently superior to others. Players who spend a lot of personal time poring over the rules can often create characters so powerful that players who don't optimize feel useless in games with those who do.
The D&D v3.x family was published under an Open Game License which encouraged third-party material. Two editions later, there is still a thriving market creating new 3.5-compatible resources.
When Wizards of the Coast discontinued D&D v3.5 development in favor of 4th Edition, Paizo took advantage of the Open Gaming License to publish an updated revision of the 3.x rules under the title Pathfinder RPG, which is still their flagship product. Notably, Pathfinder RPG has consistently outsold 4th Edition.
The D&D v3.x family is designed around building characters, monsters, magic items, etc., from small simple pieces. For each piece, any of countless others can be substituted, allowing near-infinite customization and variety.
Due to countless situational modifiers, players and DM alike have to put a great deal of thought into decisions about position, order of actions, etc., to the point that six seconds (one combat round) of "in-game time" frequently takes half an hour or more of play time.
All of the deep-dive rulebooks provide the option for endless complexity - go as far as you want, and you'll never run out of opportunities. However, the basic mechanic of "roll a d20 and add your bonus" is simple enough that the DM can tell stories, create worlds, and build adventures with their party.
3.5e is the perfect edition.
Though most other editions of D&D assume miniatures are used for combat, they have all provided rules for playing without them. 4th Edition absolutely requires a grid, to the point that distances and ability effects are defined in terms of "squares."
Making it fun was such a priority in 4th Edition that it frequently trumped making sense. This manifests in ways ranging from "minions" who die in hordes at the lightest touch, to broken weapons simply repairing themselves.
As a grid-based combat game, 4th Edition offers well-balanced and tactically rich gameplay. In particular, it features numerous effects which force or prevent movement, often preventing simple "charge and attack" tactics.
4th Edition downplayed simulation, and instead embraced cinematic action. The players are assumed to be the main characters of a story which revolves around them, rather than merely one faction in a far larger world.
Compared to prior editions, 4th Edition D&D has dramatically simplified rules. There are very few special cases to remember, and all character classes (Fighter, Wizard, etc.) share a common pattern in how they gain and use combat abilities.
Virtually all of the game's customization options are based around combat. There are very few utility options for non combat situations, which leaves other styles of play in the dust.
To encourage boldness, characters are far harder to kill than in any other edition. This also allows players to invest heavily in the story and personality of their characters without facing the potential buzzkill of loss.
Other versions of D&D tend to use a "balance over time" system, where some character types are weaker to start with and stronger later on, and vice-versa for others. However, this works poorly if your game doesn't continue for a long time, or starts with PCs already at a higher level. 4th Edition is the first - and so far only - version of D&D to do a good job of making characters similarly powerful at all levels.
Virtually most of the game's customization options are based around combat. There are very few utility options for non combat situations, which leaves other styles of play in the dust.