In my previous post, I mentioned "Rulings, not rules". You might want to go read that post first, but it's not mandatory.
It is my understanding (by looking at the OSR Primer, various retro-clones and some of the original books in pdf form) that in the old days, the rules were really "sparse", lightweight, sometimes they made no sense. You had a structure, some stats. Players described stuff, the DM described stuff. Sometimes you had a rule for what happened, but a lot of the time the DM just decided, sometimes he had you roll some dice and decided based upon that, or he rolled on some random table or whatever. Even if the number of rules increased with each supplement and edition, you were still kinda expected to use the game in your own ways
(Intermezzo: even with the infamous Gygax mandate that D&D is only D&D if played by the official rules. But that has to be taken into the context of tournaments and "compatibility" of groups and players everywhere.)
These days* you have a rules bloat, with pages upon pages of rules covering every possible situation. Character abilities tie into that. You can't just "make rulings", because there's a 90% chance there's already a rule for that somewhere in a book. The rules are the players' safety zone, they build their characters based on and through the rules. They are the player's resource to achieve something in the game. They're prescriptive (Players: "I do X, and Y happens."), instead of descriptive (Player: "I do X.", DM: "Q happens."). If you're making rulings, you're taking away power from the players, cutting their reliable resources (or currency regarding those resources). For more on reliable vs unreliable currency, check out this post from Vincent Baker (for which this might be required reading).
In a game like oD&D, rulings are mandatory, they're part of the game. In a game like 3.x or 4E, rulings are deprotagonizing a lot of the time. I could try to poke at the issue of why and how this came to be, but maybe another time.
*When I say "these days" I am talking about "my" version of D&D, which is 3.x/Pathfinder. I have zero gaming experience with 4e, although I have read the books, but the situation doesn't seem much different. If anything 4E is even more prescriptive than 3.x.