In the post TSR era of D&D (2000+) there has been a lac of the D&D feel or essence in D&D. It is not so much the mechanics as I have just recently bought an AD&D retro clone with d20 mechanics. Here is my opinion of mechanics, the balance brigade and numbers in general.
"Reducing a character to a list of dice rolls and modifiers is not role playing"
pg 30, AD&D 2nd Ed DMG, 1989.
Put simply I do not really care if they use an attack matirx, THACO, BAB, weapon proficinecy etc, not to worried if they have NWP, skills, ability checks etc. Put simply I care more about the fluff and the feel of previous editions and as long as the mechanics are not completely awful and they do not repeat CoDzila I'm not to fussed. The essence can be hard to nail down so I will try and define a few things.
When I open up the PHB I want a +5 holy avenger to land on my left foot. A real holy avenger and not the watered down post TSR versions of one. I want elves with dark vision. When I open the DMG I want a +3 frostbrand +6 vs fire using creatures that functions as a ring of fire resistence to land on my right foot. If the PCs kill a Drow and take their weapons I want them to have to worry about them falling apart in sunlight. Beholders with anti magic central eyes, Yuan-Ti slithering through the jungles, Githyanki gish riding red dragons through the astral plane perhaps getting ready to invade the prime material. Fireballs dealing 1d6/level would also be nice regardless of the other mechanical changes.
The ghost of Gygax guiding the d20s also works. A game that pays tribute to Gygax, Dave, Frank Metzer. Also other designers, artists, and writers of years past. Hickmann, Weiss, Elmore, Easley, David 'Zeb' Cook, Eric L Boyd, Greenwood, Brom, Sean K Reynolds, Monte Cook, Salvatore, Elaine Cunningham, Roger E Moore, Troy Denning and numerous others whose names elude me. Sure there is an AD&D bias there but that I feel is the essence of D&D- its history, legacy, monsters, spells etc. I do not care so much about the AD&D mechanics except maybe as far as iconic things like magic missile, fireball, and certain magic items are concerned They do not have to be 100% clones of AD&D mechanics as something like the effects of a ring of fire resitance can change between the editions. If you find the Sword of Kas one should have a reasonable idea what the sword will do because it is the friggin Sword of Kas and not some generic +6 sword. Not that I am opposed to a generic +6 sword but a +5 one still evokes earlier editions. A +5 sword used to be special.
Bounded Accuracy, not a bad concept to reign in the number porn of 3rd and 4th ed but I want a +5 holy avenger. Stretch it please to make it work. a +3 one doesn't feel the same. Not to worried if a cleric or priest can fire laser beams, not to worried if you have a non Vancian Warlock or a sorcerer. Not to worried if a high level fighter can cut peoples heads off without a vorpal blade but has been trained at court and has social skills. Not to worried if "Spells and Magic 2.0" has an AEDU wizard in it or if the Warlock is AEDU, the 3.5 version or something else. I prefer monstrous races to be kept out of he PHB but if one is going to add the Tiefling and Dragonborn in there may as well throwin a few more races as well like Aasimar and Genasi. I'm not even that worried if the Ranger can dual wield or not or have an archer build or has 2d8 hit dice at level 1. The classes have to feel familiar though so nothing to weird (Paladins growing wings and lobbing fireballs, a multclass tiefling or aasimar one though maybe).
And thats more or less it and it is what I am looking for.