Over on ENWorld, I managed to get myself embroiled in a debate about the value of 'hidden' rules - those rules in the game that aren't immediately obvious but which only become apparent with system mastery.
It's fair to say I'm not a fan. Frankly, I'm of the opinion that if something is important enough to be a rule it should be spelled out in big, clear letters; and if it's not important enough to do that, it's not important enough to be a rule.
Now, it's worth noting that a rule may be 'hidden' for several reasons:
A rule may be hidden because the game is badly organised. This is quite common in early editions of the game, where things were gradually added as the game was crafted, largely on the basis that they were 'cool'. I'm inclined to think that if there are rules hidden due to bad organisation of the rulebooks, that's a design flaw that should be corrected up at the next reasonable opportunity.
A rule may be hidden because the rules-text is badly phrased and inpenetrable. The classic example of this is the rules for initiative in 1st Edition, where it's entirely possible to read the text carefully, and several times, and still not understand what it's getting at. (See also the 5e rules for creating monsters.) Again, I'm inclined to think that any such instances are a design flaw, and should be corrected at the next reasonable opportunity.
A rule may turn out to be hidden because there are in fact two contradictory rules and it's not clear which takes precedence. Or a rule may turn out to be hidden because there are multiple rules in play and the interactions (or order in which they should be applied) is not clear. And, sure enough, I'm inclined to think this is a design flaw. The authors should consider the situation, work out what they intended, and clear it up at the next reasonable opportunity.
But all of those are basically just mistakes that can, and should, be fixed when possible. What really annoys me are those cases where the designers have deliberately hidden rules away, as Easter Eggs to reward those players who develop "system mastery".
3e was probably the worst edition for this. A good example was that they made the longsword the best melee weapon (in 3.0e - it changed in 3.5e), because it did the most damage for a one-handed weapon (allowing a shield), and had a 19-20/x2 critical which meant that you tended to get less 'wasted' damage if you rolled a crit against a low-hp opponent.
There are two things wrong with this thinking, though. The first, and lesser, problem is that the designers didn't actually have as much control as they thought they did - the fact that the longsword was better than the battleaxe was a trivial nonsense next to the difference between 3e casters and non-casters - who cares about marginally more damage when the Wizard can move mountains with a word? And for all that the designers have very cleverly built in this 'best' melee weapon, they seem to have utterly missed that cheap magic item creation has the consequence of removing the constraints of Vancian casting - the only thing keeping those casters in check.
But the bigger problem is that the designers didn't seem to consider that adding hidden rules, and rewarding system mastery, in this manner makes the game harder for the DM to run. And the DM isn't a competitor in the game, nor a competitor with WotC (in any sense) - one of the purposes of the DMG should be to make the game as easy as possible for the DM to run. If there are any hidden rules there for the players to find, those same rules should be spelled out very clearly in the DMG, complete with their explanations.
Another example of a hidden rule in 3e perhaps explains this better: there's a convention in the spell design that a spell either requires a touch attack roll or a saving throw (almost never both). Spells that require a touch attack will almost always hit, since touch ACs are generally poor, and so they do "1dX plus level" damage. Conversely, spells that require a save a much less likely to hit, and so they do "XdY" damage - they're vastly more damaging. But this convention is never actually spelled out, even in the DMG where it talks about creating new spells.
(And, consequently, when later designers started adding new spells, they seem not to have known about that convention, and therefore tend not to apply it - with the consequence that many newer spells are simply better than the PHB ones.)
Or, again, there's a convention that spells that do ability score damage (such as ray of exhaustion) can't reduce a target's ability scores to 0, since that would lead either to death or complete incapacitation on the part of the target. But, again, this isn't spelled out, and so there's a spell in the "Spell Compendium" that does massive Dex damage with no cap, and therefore allows mid-level Wizards to trivially one-shot dragons.
Oops.
(There's also the Wealth by Level table, which for years led to DMs being accused of not giving out enough treasure, indeed to the extent that Pathfinder now encodes that DMs should give out a 'right' amount of treasure. Only it turns out that WbL wasn't any sort of target amount, but rather just the likely outcome of using the provided treasure tables. And, indeed, DMs certainly shouldn't have let new high-level PCs start with the value given on the table, but more likely around 60% of that value. Though in that case I'm not certain whether that was a deliberately hidden rule or something that was only realised later.)
Now, it is worth noting that, of course, there will always be some hidden rules. The complex nature of the overall ruleset, coupled with the fact that the designers are human and therefore imperfect, guarantees that this will be the case. So I'm not foolish enough to request they be removed entirely. However, while I'm not going to call for perfection, I think it is reasonable to ask for better - and to keep asking for improvements as time goes on. After all, wouldn't we all like a game that is exactly the same... but better?