What one thing?

[Ahnehnois]

Included:
Standard modifier.
A common mechanical platform for all dice rolls (i.e. closer to the standard modifier than the different scales for BAB, saves, and skills that 3e has).

Excluded:
The power system.
Fighters with spells. Nonmagical magic. Arbitrary daily and "encounterly" limits on activities. Common system for all class abilities.

Returned:
Multiclassing
I'll have to echo the OP on this one. Picking class levels one at a time was one of the best D&D rules decisions ever made. If you're going to do classes and you're going to do levels, this is the way.

 

[AeroDm]

Include: The math. People have griped about 4e's commitment to the math but I think it does a greater service than disservice. It is easier for the GM to fix a +/- 2 error than a +/- 8 error.

Exclude: Powers. They were a neat experiment, but I ultimately am not a huge fan. They vary so greatly that it feels like players have a hard time choosing between them and that slows down the game. When a player was deciding between "Power Attack for 4" and "Power Attack of 5" it went quick but was still tactically important.

Returned: Multiclassing. I've harped before about how in 4e you get to pick your character but in 3e you got to build your character. The distinction is small but it was a big part of the game for me and I miss it.

 

[diaglo]

1. nothing
2. all of it
3. reintroduce OD&D(1974)

 

[Pentius]

Include: Powers, or a variant thereof. The basic idea that even a non-caster should have interesting, viable combat options and tactics on par with casters, without resorting to DM fiat.

Exclude: +X magic items. Or any variant of an item need to make a character's math right. It may be a bit counter-intuitive, but I think it would go a long way toward separating "having attacks and defenses on par with your level" from magic items, thus making items rarer, and optional, thus helping the feel of 'magic'. The inherent bonus system kinda does this, but still.

Returned: Mechanically, I can't really think of anything that hit the cutting room floor that I'm not glad to be done with, so I'll use the cop out answer of "Increased support for (non-FR)Published Settings."

 

[pawsplay]

Include the new elemental cosmology
Exclude, uh, I'm going with with huge laundry lists of powers for every class
Bring back the wacky spells

 

[Herremann the Wise]

Keep: 1. The mechanical focus on teamwork.

Get Rid Of: 2. A lot of simplistic, gamist stuff be it wonky powers, saving throws, rules and other stuff that still irks me every time I play. Skill challenges should be given the arse too - too many times they just don't work and come off weak sauce. And the weird alignment system while we're talking about it.

Bring Back: 3. The Character and their class(es) having a greater impact on play than their role. Less focus on combat.

Best Regards
Herremann the Wise

 

[Stormonu]

Keep: Static defenses

Lose: Powers

Return: Monster stats with abilities beyond the encounter

 

[Dannyalcatraz]

1. nothing
2. all of it
3. reintroduce OD&D(1974)

To thine own shelf, be true.

 

[Dannyalcatraz]

My issues with 4Ed are numerous, but playing by the OP's rules...

Keep: Rituals, except better done.

Lose: The "powers for all classes" system.

Return: REAL multiclassing. Hybridizing is OK, but I find it still lacking. It's more like gestalting.

 

[FireLance]

Include: Powers, but no need to slavishly stick to the AEDU model. Essentials makes me hopeful in this respect.

Exclude: The current saving throw as a mechanic for determining the duration of effects. I find it too simplistic and would prefer (say) a defence roll against a DC dependent on the attacker's level or attack bonus instead.

Return: Really cool magic items, artifacts that aren't intended to disappear after a few levels.