hmmm. Well, How&Why's list is most complete.
I would reiterated XP and Class since it seems there is an increasing movement who want a "classless" and/or "XP/level-less" D&D...judging characters, instead, based on acquisition of feats and skills and powers.
To me, you throw those out....you really have no right to call it "D&D" anymore.
I would want also to see a return to "Vancian magic" (gods I hate that term, but it is the quickest/easiest term for the "fire and forget" method of spell casting).
[EDIT 2] Maybe all of the flack Vancian magic gets is because of the name? The branding? If you toted it as "Classic casting/magic" or "Studied casting/magic" it would be more tolerable for people.
In example, I had all "A's" in high school (and most of college). I could take a test in Chemistry, Physics, Mathematics, History, etc...and ace the test. Because I studied! If you asked me to do it now, I severely doubt I would actually PASS let alone get an "A."
The innovation of the "Sorcerer"/spontaneous casting classes are beloved by many...most, I would wager to say...but is it a "core" archetype? Morgan le Fey, Merlin, Soloman, even Disney's "Witches" (though, technically, Maleficent was flavored/described as a evil "fairy" like/working against the 3 "good fairies") were all consulting their occult grimoires. [/EDIT 2]
Spontaneous casting in "core/basic" classes, buh-bye. Prestige, or whatever they decide to call them, fine. But not for the beginner/basic "arcane spell caster" class.
Also, I know this will probably get me shot, mugged, strung up, drawn and quarter, and dunked in the river (or at least "boo-ed" off of ENworld) but "THAC0" was not really all that bad.
A single number your character had, that changed with level, and could deduce your roll to hit. No more big heavy charts for everyone. One chart per class with one number you needed to known/mark down.
I really never understood/understand all of the flack THAC0 gets.
Is doing subtraction
really so difficult in today's day and age? Your opponent's AC is "X". THAC0 - X = what you need to roll on d20.
Of course, that should/could be flip-flopped, seeing as pretty much everyone likes "increasing" AC instead of "lower=better" (myself included). "Higher = better" just seems more intuitive for everyone/an improvement.
So, flop the chart. You could have a THAC0 + X = roll needed to hit. Easy-peasy.
Obviously, this would require a reworking of what counts towards AC, how you determine AC, where AC starts (0, presumably), etc.
Roll of 20 still always hits, of course! That is also a "sacred cow" I couldn't see losing and still calling the game "D&D"...just because someone can claim, "I have an AC of 26! You can
never hit me!" and a roll of 20 with only a combined +4 to your "to hit" (whether it is via feats or powers or racial abilities or what have you)
would never touch them.
No "double roll" needed. No "crit check"...Just, "Rolled a 20? You hit (period). Double damage (period)."
But a return to the "THAC0" mechanic would not "turn me off" of DnD:TNG.
[EDIT] OH! And Races. Oy. A return to a "simplified" list of races.
Ok, Unearthed Arcane (1e's, I mean) opened up all of the "sub-races" of elves and dwarves and gnomes (I never considered subraces of "halflings" to be legitimate. Give me hairy feet of give me another race!

). Ok, that got a little ridiculous for number of races.
But it pales in comparison to the "Eladrin/Dragonborn/Tiefling/Drow" onslaught that was to come.
Half-dragons, Half-elementals, Half-demons, Half-angel-half-giant-half-gnoll ("Cuz I'm a character an' a half!"

), Goliaths and Wilden and Shardminds...oh MY!
Pick...what did we originally have in 1e...7?
Give me 8...maybe 10? That's all a "core/basic" game needs.(Though I'd make the argument that years worth of fun gaming is doable with just "Human, Elf, Dwarf, Halfling") Everyone else...you have them in your world? You like them? Fine! Introduce all of the NPCs you like...or homebrew your own PC race out of the "non-core/rules" races. We did it for YEARS without problem...Nixie PCs, Were-whatever PCs, Doppleganger PCs...none of these have to be "RAW" races, imho.
Ok. I think that's all I have on that.
No matter what it has or doesn't have, have fun and happy gaming and best of luck with whatever DnD:TNG brings.
--Steel Dragons [/EDIT]