1e is both malleable and forgiving when it comes to kitbashing and rulings. It's the one true advantage of what I call its underlying mechanical chaos.
I don't think so. As the lockpicking example illustrates, it's very easy to make a ruling in AD&D that runs roughshod over some other, carefully rationed, class ability.
But the tone of the PH* - the player-side info - in both 3e and 4e is "here's what you're allowed to do, and here's where the borders are". There's no real encouragement to try anything else because it's presented as if there's a rule for everything.
4e PHB pp 6, 8-11.
D&D is a fantasy-adventure game. You create a character, team up with other characters (your friends), explore a world, and battle monsters. While the D&D game uses dice and miniatures, the action takes place in your imagination. There, you have the freedom to create anything you can imagine, with an unlimited special effects budget and the technology to make anything happen.
What makes the D&D game unique is the Dungeon Master. The DM is a person who takes on the role of lead storyteller and game referee. The DM creates adventures for the characters and narrates the action for the players. The DM makes D&D infinitely flexible - he or she can react to any situation, any twist or turn suggested by the players, to make a D&D adventure vibrant, exciting, and unexpected. . . .
When you play your D&D character, you put yourself into your character’s shoes and make decisions as if you were that character. You decide which door your character opens next. You decide whether to attack a monster, to negotiate with a villain, or to attempt a dangerous quest. You can make these decisions based on your character’s personality, motivations, and goals, and you can even speak or act in character if you like. You have almost limitless control over what your character can do and say in the game. . . .
Your "piece" in the DUNGEONS & DRAGONS game is your character. He or she is your representative in the game world. Through your character, you can interact with the game world in any way you want. The only limit is your imagination - and, sometimes, how high you roll on the dice. . . .
The Dungeon Master decides whether or not something you try actually works. Some actions automatically succeed (you can move around without trouble, usually), some require one or more die rolls, called checks (breaking down a locked door, for example), and some simply can’t succeed. Your character is capable of any deeds a strong, smart, agile, and well-armed human action hero can pull off. . . .
How do you know if your sword-swing hurts the dragon, or just bounces off its iron-hard scales? How do you know if the ogre believes your outrageous bluff, or if you can swim the raging river and reach the other side?
All these actions depend on very basic, simple rules: Decide what you want your character to do and tell the Dungeon Master. The DM tells you to make a check and figures out your chance of success (a target number for the check).
You roll a twenty-sided die (d20), add some numbers, and try to hit the target number determined by the DM. That’s it!
There's probably more I haven't got too, but that seems enough to show that your view of tone is not the only one, and not terribly warranted by the actual explanatory text of the game.
All 1e Fighters may be mechanically the same (or very close), ditto for all 1e Thieves; but the 1e Fighter and the 1e Thief sit on very different mechanical underpinnings from each other. And this is what I'm getting at; mechanical difference and disunification between classes, rather than between individuals in the same class, greatly helps define each class for what it is - and isn't.
I mean more in terms of underlying chassis. Some examples:
Different level-advance rates a la 1e-2e;
Different mechanics e.g. Cleric casting mechanics are not the same as Bard casting mechanics are not the same as Wizard...;
Knights, Paladins and Cavaliers use a different combat matrix in honourable combat (or tournaments) than in mass melee;
Multiclassing works differently (or not at all) depending what combination of classes you're trying to combine;
Some class-skill combinations use d%, others use d20, sometimes it's roll-over, sometimes it's roll-under, etc., depending on the level of granularity and intended outcome probabilities required;
Etc.
And then there's resource management: does everyone get their spells and-or hit points back at the same rate, etc., but that's at a different level than what I'm thinking of.
As I've already posted, when I think about what differentiates PCs in a RPG,
resource recovery rate is not what I look for. That seems more of a wargame priority than a RPG priority, to me at least. And this is even moreso for "underlying chassis"/underpinnings like acquisition of abilities, or which dice to roll for resolution.
The key in a RPG, for me at least, is the way characters affect the fiction. Which goes back to
@Charlaquin's remarks way upthread: in a RPG which makes combat a significant part of the fiction (and 4e DMG is definitely one such) then
pushing your enemy into a pit or even just
driving them before you is interestingly different fiction from (say)
withdrawing and luring them after you or
throwing your shuriken and blinding them. The fact that the resolution and resource framework is broadly similar isn't an impediment here but a virtue, as it makes interpreting and applying the rules easier.