Something Old, something New, something Borrowed, something blew...

[mkill]

I think the answer is to consider game elements on their own merits, not by the year / edition they were introduced.

OD&D: Simple core. 5E needs a basic box that contains about as much stuff as the original white box.

AD&D: multiclassing. There is something dead simple about the concept of "pick two classes, get both, if something is in both classes, get the better; in exchange you level up slower".

AD&D: A dose of wackyness. Cursed items, deck of many things, wish spell.. It shouldn't be central to the game, but later editions suffer from taking themselves too seriously.

AD&D2: Beautiful, detailed, well-supported settings.

3E: Tons of player options. Pick every monster. Pick any of 700 prestige classes. It's just turned up to 11. Now, 5E shouldn't repeat the full insanity of it, but it definitely shouldn't be afraid to support exotic PC concepts.

4E: combat roles. Don't match each class 1:1 to a role, don't make the role define class features, but keep the concept. Give each class a useful part to play in combat. Deliver the same for exploration and interaction.

4E: Rituals. Make them cheaper regarding time and money, but keep them.

4E: At-will magic (the 3E Warlock already had it)

Feng Shui: Anything goes combat and supporting stunts.

Fate: An aspect (or two) that defines something unique about the PC, and that can have positive and negative consequences - replaces action points

Pathfinder: excellent modules.

Earthdawn: Magic items with a unique legend

E6: Numeric progression that stops once you reach a certain power level

Shadows of the Colossus: Gigantic enemies that are more like terrain instead of duel opponents (I have yet to see a pen & paper system that handles this properly. No D&D edition does.)

 

[Minigiant]

Old: 3E magic weapon and armor enchantments. I just like them. I miss my quiver of bane arrows.

New: 4E separation of combat and noncombat resources. Attack powers and noncombat/defensive power did not use the same slots. Skills are not factored into combat balance. You didn't have to sacrifice all your combat ability to be good in exploration or interaction.

Borrowed: Mass Effect 2&3 class design for hybrids. Unlike ME1 where hybrids are purely half one thing and half another, ME2&3 hybrids were given features that make them into unique beings. Infiltrators have Cloak to allow sneak tactics. Vanguards aren't just "fighter/mages", Charge gives them a rush attack unique to them.

Paladins should not be fighter/clerics. They might combat strength and divine spells but something else is their real defining feature. Rangers should be defined by their Favored Enemy/Hunter Quarry. Barbarians by the their Rage. Etc.

Blew: the Hard "Full stop" Mundanely-Counterless spells of 3E.

Does Wind Wall have to block all arrows? Even Magic ones?
Does Wall of Force have to be unbreakable?
Does Knock have to be so simple to use? Not even a skill check?
Does Foresight have to be foolproof?
Does Pass without Trace have to remove every trace and make tracking completely impossible?
Why does Fly give you a free Featherfall? When the spell wears off, the target should go "Oh NOOOOOOOOOOOOO! SPLAT!"

 

[steeldragons]

AD&D: multiclassing. There is something dead simple about the concept of "pick two classes, get both, if something is in both classes, get the better; in exchange you level up slower".

Thank the GODS! You mean I'm not the ONLY person left in D&D creation that thinks this!?! YAY! Thank you!

(PS: the rest of your list was all good stuff too, but I nearly jumped out of my chair when I read this one!)

To the OP: cool/interesting way of presenting a thread. Nice job.

"In 5e I, Steel Dragons, would like to see...

OLD: AD&D Multiclassing (lots of other stuff too, but I believe the OP stipulates 1 thing, each)

NEW: What little I know of 4e, though have read scads of opinions and thoughts on different things, I think the best thing I can think of to bring in from 4e is...the IDEA, though not the implementation, of "powers" as in cool things everyone can do...the "at wills", I suppose, but definitely NOT the whole AEDU mess.

BORROWED: This is somewhat tough as I do not play anything else...but, again, from what I've read/seen/heard about, some kind of Fate/Luck point kinda system to help you out in a dire situation would seem to be kinda cool, fit in, and not be something that we've seen in D&D before. VERY limited options in what you could apply these Fate/Luck points to, but I could see them working in play.

BLEW: 3e multiclassing and the whole "taking a level" mentality that came with it.
Me to some random gamer I meet: "Oh, you play D&D! Cool. What characters do you have/like to play?"
Them: "Yeah! I have this kickASS Ranger 6/Barbarian 4/Spellthief 7/Sorcerer 3/Druid 1...who I'm gonna work up to become a Your-Priest Factorychum Arcmaggie."
Me: Uhhhhh...cool?...

 

[Mattachine]

Old: Faster gameplay during combats. More willingness to make DM rulings, use houserules, and so forth--less codification, probably because of the lack of Internet use.

4e: Balanced classes, classes that can contribute to all sorts of encounters, and the Essentials take on classes (mixing complex and simple)

Borrowed: Any system where backgrounds/themes have a roleplay mechanic--World of Darkness, Burning Wheel, and Spycraft all come to mind.

Blew: 3e simply made magic too easy and too powerful

 

[avin]

Old: Planescape.

New: Faerie and Shadow as parallel planes of the material plane ( not "Feywild and Shadow parallel of the World").

Borrowed: World of Darkness backgrounds.

Blew: linear fighters vs quadratic wizards. That must die.

 

[Wednesday Boy]

Old: I really enjoyed how 3.x prestige classes and regular classes were generous with class features. Class features that can be used any time feel like an embodiment of the character's abilities in a way that a refreshable power doesn't.

4e: I love playing a martial character who can do interesting and fun things each round, like a spellcaster can.

Borrowed: Hands down, aspects from FATE. I love those things. In our upcoming D&D game we've discussed adding them.

Blew: 3.x full attacks and five foot steps made combats supremely boring once meleers were engaged in combat.

 

[Man in the Funny Hat]

OLD
A bit of a vague concept for this: I want to see an ease and rapidity of character generation. A player should be able to walk into a game cold and have a PC ready to play in 10 or 15 minutes. And that does NOT mean relying on software to handle the complexity. Mostly that means not having to spend hours or days reading, comparing and contrasting skills, feats, class abilities to be able to make meaningful character creation choices.

NEW
Actually, that would be Healing Surges. I just think the name and implemetation sucked extremely. The principal, I believe, can be adequately flavored and constructed to shift away from an excessive reliance upon healing spells, potions, class powers. Mostly I like the idea because when I remove healing ability as a focus for justifying the existence of clerics they become VASTLY more interesting.


BORROWED
I don't really play non-D&D RPGS. Certainly FAR too few to be able to poach rules from them. I'll go a little vague again and say that while I have no issues with Vancian magic I do have issues with the quadratic nature of spells. Spellcasters, indeed ANY class need not be "balanced" as such but they do need to be fun to play and neither overwhelming nor underwhelming at any given level.

BLEW
Easy peasy. 1E AD&D - initiative system.

 

[El Mahdi]

OD&D, BD&D: Simple basic core system and character generation

AD&D (1E & 2E): a benchmark for the first stage of complexity above basic system, and Kits! (which have already seemed to have been mostly fulfilled by themes and backgrounds).

3E: the ability to model just about any archetype or idea you can possibly imagine, and use this system as the baseline for the upper limits on granularity and complexity (Skills, Feats, Race, Classes).

4E: balanced math (system), front loaded Races and Classes, ease of prep, At-Will spells and combat maneuvers, Rituals!, a modified Healing Surge mechanic (more for use in between combats only, and for use with the Heal Skill).

 

[Boarstorm]

OLD: 3E Multiclassing, hands-down. Nothing helped me build a character mechanically that matched my vision thematically as well as this. It had its issues, but for non-archetype characters, it was as close to ideal as I've yet encountered, while sticking to a class system.

NEW: Dynamic combats! Rituals were also in the running here, but ultimately the award had to go to all the little changes that injected movement into battles. The days of standing still and exchanging full attack actions are over.

BORROWED: VP/WP from the first d20 Star Wars game. I love them. The crits-straight-to-WP kept the game exciting and deadly, even at high levels. Thematically, it didn't really fit Star Wars well, imo, but I think that's the worst that can be said for them.

BLEW: The Magic Item Treadmill! I want off!

 

[dangerous jack]

Old: 2e multiclassing (although let me pick up a class later on and just spend the XP), and the relatively compact size of the player's book from BECMI

New: Second Wind (although I like the surgeless 50% per encounter ala Gamma World)

Borrowed: I don't really play non-D&D, but I like the idea of Savage World Trappings

Blew: From 4e (and 3e), the ability to exchange standard actions for moves (not to mention moves for minor actions). This leads to characters that can cross full battlemaps in a single turn.