D&D, Rado's take on it

Abstraction, realism

It's very abstract: means very far from detailed simulation of real world. Even though rounds have been cut down to 6 secs, this doesn't mean it's anything more "real". It's just faster, if you will "more realistic" for the matter of overall combat time.
Never argue "the single mechanic xyz is broken from the real world". The system exists as whole. The goal is not to be correct in details, but have a working, easy "fun" system to run.
Most things make sense, you just have to understand the connection between each of them, and how each of them is explained. Sometimes it sounds "against logic", especially when talking about order of events, but always keep on mind that to make it easily playable you have to pay some drawback, which sometimes is reality or forward logic, where it uses "backward" logic.
D&D is about resolving situations and effects. It does not work with single actions: i.e. single word swing. It works with series of swings and feints within a round, which allover is expressed in a single die roll (for attack, and another for damage in case of success). This is most important !
Reverse logic: result/effect comes first, explanation with detailed elementary actions why/how later given by DM/players. That's ok because DM and player play/work TOGETHER. Result know before action, depends on result. In other normal games it's the players trying to overcome limitations set by DM and left alone by him. There (or when you don't trust their sense of roleplaying== explaining things, physical and abstract) to figure out the players contribution you want the players decision BEFORE he knows the result. Action given first before result, depends on action. Dice are for simplification and speed. If you can handle something sanely, do it without them. Dice are for fairness between DM and player. If you trust either side, do it without them. If you want to have a (fair) chance independent of trust, use them ! There is always a chance ! That's what the game is about. More dice rolling != more realism, just more randomness. A good model can work with 1 roll, a bad model will still be bad even with 10 rolls. Keep it simple, but fair.

common or not common

Some things are common to a fantasy world, others are not. Those that are, the DM should operate automatically unless player chooses differently.
Declare what is and what is not common. Example: Magic -> saves. If magic is common, people save against magic in different ways. If practically there is an exceptional but still common way to save, it happens. If it's not, then require players to make their decisions. Similar goes for food, weapon maintenance, buying equipment ...

Attributes, skills, saves, actions

3 mental, 3 physical, analoguous functionality: offense(active), defense(passive), mobility(transfer). Same funtionality -> same actions -> save variation with a single will base score to immitate 2 saves for the physical world. -> general saves variants to cover "special" cases. Atributes summary of abilities until starting age. ranks: special training by dedicated resources. AB, save bonus: special training through the "main trade": adventuring Again: these reflect abstract complex results to succeed or fail a given (complex) situation. None of them is tied to detailed SINGLE specific actions. saves are beyond player control, too fast: first save, then what happens. Certain mechanics allow for player input, others don't. Saves don't. Basically: anything that is to fast to ponder is automatic by the PC, not player controlled. If player controlled: before a save ask: what do you do ? and always wait for answer: no answer, no save. Should for all saves be allowed to speak up, not just when DM feels like it.

It's not the DM's game alone.

if it is: decisions by DM's whim, bends given rules to make his story. if it's not: DM just controls rules of magic, physic, social, but they don't change without notice to players. Same chances for DM and players alike to do with their N/PCs as they wish. The DM assign chances for the PC, not for the player. Assign to high, bad luck assign to low, good luck assign correct, fair Whatever the case, this will be the reality for my PC. The reality is not defined alone by what the player wants to do. It's defined by the capabilities of the PC, not the player. Especially without conscious choices (example saves). Whatever goes beyond player will, the DM has to take over and cover up, even for the player's PC. DM controls reality in world, including PCs. Players just add direction where they _can_ do so. However, they are not _required_ to do so.

standard, special, ...

it covers ALL cases, standard and non-standard, special and non-special. Especially special abilities means "special to the general world", but not special to the being in question: for it it's normal. -> automatic use in saves. If you wish to respect exceptionality, it offers "optional" rules that are in line with the rest of the system: you NEED NOT make up own for what is covered. When you think there is a gap, introduce a new fixed rule that applies always the same, not by whim: general abstract, not detailed specific. No need to complicate the mechanics for non-standard or special cases. Have more inputs to vary chances, but keep the mechanic simple, the abstraction level the same.

Consistency

Challenge as rpg means changes and variety of events for the PC in the world. BUT, as player need consistency with the real world, i.e. rules of the rpg-system, they should not change. At least not without notice of players, even if doesn't affect them currently. It might at unexpected times. Only then we can trust in the DM to run his world consistently. Trust comes from knowledge of the past with the hope that it projects similarly into the future.