A more detailed, all-encompassing mechanic like d20 also has benefits... fewer mechanics require a judgment call. I also found that there was a lot less rules-lawyering in 3e (at least, in the early days) because the rules were straightforward and applied everywhere.
I agree that one of the things I enjoy about 4E (or 3.X with a group that has a high level of rules-expertise and has been playing together long or well enough to work out consensus) is that it's clear to everyone what will happen in a given situation. When I DM, I can rely on the players to remind me how the rules apply to this situation; when I play, I can point out rules that the DM might have missed. Individual exceptions exist - some people just want to argue, or seek personal advantage - but it's almost always the case in my experience that a new-school game can achieve a friendly feeling of communality, where the rules are maintained equally by everyone & apply fairly to all.
It's worth pointing out that this new-school advantage works directly against the idea of rulings instead of rules. Because the outcome of the rulings aren't agreed upon by everyone ahead of time, it opens the possibility of unfairness. There's a strong incentive to stick with the rules as written because that keeps you in the safe harbor of the communal understanding of how things work.
Let's say that an efreet is holding a captive on a bridge and planning to run across it to immolate the captive in a lava pool. One of the players uses the
command spell to say "Drop!" In 4E, the effects of the spell are sharply limited to avoid judgement calls; the efreet is either dazed or pushed, as the player desires. In 3E, the effects are similarly pre-defined: "On its turn, the subject drops whatever it is holding. It can’t pick up any dropped item until its next turn." But in AD&D (or OD&D Supplement I), the rules don't define the effect (other than that the subject must obey), so a ruling is required. The efreet might drop the captive, or drop to the ground like it was about to do pushups, or step off the bridge so that both it and the captive drop into the chasm. If the outcome isn't what the player wants, they might feel they're being cheated by the DM, discriminated against if the rulings always seem to go against them but in favor of other players, etc.
Personally, I prefer the expansive possibilities of a spell that can make someone obey any one word you choose, and feel something is lost when it's restricted only to "daze or push". So I'm willing to pay the cost of giving up the pre-agreed rules framework in order to gain the flexibility that comes from rulings, but I think it helps to be aware that there is a tradeoff. When I DM OD&D I'm wary of creating the perception of unfairness so I try to make sure that we build a consensus around each ruling. I might ask the players whether anyone remembers a precedent that would apply here. Or I might say "Which outcomes do y'all think are most likely?" and then leave it up to the dice, so that it feels like an objective thing instead of fiat on my part. On a 1-5 it might not go the player's way, but the dice might always come up 6, and leaving that possibility open means that we don't have to argue endlessly about whether that interpretation is absolutely possible or impossible, just more or less likely. I'm also aware that players with a new-school outlook may feel disempowered by not knowing ahead of time what the outcome of their action may be, so I'm open to someone deciding not to take an action if it looks like it won't turn out the way they intended.
I enjoy the process of coming up with rulings - it gives me a feeling of ownership of the game, and gives it versimilitude because very unique situational factors that you'd never have rules for are easily reflected in rulings. By sharing the process of arriving at the rulings with the players, I avoid hogging all that fun and also feel less insecure about my rulings; some of the burden of making wise decisions is put onto the shoulders of the players and the dice.
