Plaintiff wrote...
What are "role-playing mechanics"?
Sylvius the Mad wrote...
... the amount of autonomy a player's character can comfortably have within the framework of roleplaying is zero. The extent to which a player can accept [character] autonomy is the extent to which the player isn't roleplaying.
(Brackets mine) There is your essential roleplaying rule. Any RPG mechanic must allow the player to have all the autonomy to determine the character's behavior, and the character's behavior, not the player's, should be the only thing to affect the game world. The extent to which a player's skill is involved in shooting in ME, for example, is the extent to which the player is not roleplaying.
How the RPG mechanics model the character and it's behavior is immaterial. Levels are merely a way to reflect character growth. There are other ways to model this. There doesn't necessarily have to be any progression at all, if it's reasonable to model a character who never improves at anything.
Rolling a d20 is merely a way to randomize the character's interaction with the game world. Again, replaceable. Randomization isn't necessary, either, as you can have predetermined consequences for any action.
Hit Points are a gamist abstraction, and are certainly unnecessary, though they're a relatively easy way to model fatigue, wounds, or relative well-being. But, if this isn't relevant to the particular game, it can be discarded.
Attributes reflect the character's abilities relative to the rest of the game world. They define
degree of skill or ability. They are useful, but not necessary, as it is possible to roleplay a character in a setting in which relative strength or dexterity may never be an issue. Of course, if your character is swinging a sword, then strength
is probably relevant, so the game should model it where applicable.
In other words, there aren't really any specific RPG mechanics which are of themselves necessary to make an RPG, so long as the ones that are used model the character and not the player, while allowing the player absolute control.
This, as Sylvius has discussed, does
not mean absolute freedom to do anything conceiveable. If a player only has two choices he or she still has absolute control so long as the decision belongs entirely to the player.