Njall said:
To me this reads a lot like "we call it a guideline, but you'd better follow it or balance is thrown out of whack".
Sure, we can Rule 0 just about anything in this game. This doesn't mean that it's necessarily a good idea

( or, in this case, that it will work anyway)
Yeah, this is the thing a lot of people fail to take into account when they Rule 0 something out. The 3e rules were designed as a whole, and the the rules "talk" to each other way more than they did in 2nd or 1st. And I anticipate this becoming even more tightly integrated in 4th.
The idea is that if you want to create a balance (between different PCs, between the PCs and the enemies, between the PCs and their environment) you need to carefully control the numbers involved in the game.
The 1st and 2nd edition game design philosophy appeared to be closer to "design each rule by itself and see how they worked together afterwards." For instance:
1) Fighters, Paladins, and Ranger should be the best fighters, so lets give them the best THACO.
2) Since there is magic, people would make weapons that are magical, and +1 through +5 would be a good range.
3) This monster should have slightly tougher hide than a rhino and a rhino has an AC of x, so we'll make this one 1 lower.
4) It would make sense for people to make magic items that make you as strong as a giant. Giants have 20-25 strength, so the items give you that.
The 3rd Edition design appears to have started with the 2nd edition numbers as the BASIS for everything, but then proceeded to try to patch some of the "holes" in the system. For instance:
1) In 2nd Ed, some DMs were handing out +5 items at 3rd level, others +1 items at 20th level. It caused a lot of players to complain that monsters were WAY too easy or WAY too hard. Plus, DMs were complaining that they didn't know what were appropriate magic items for different level characters. So, we create the wealth by level guidelines and price out the items so they can only be afforded at the levels they are supposed to be at.
2) In 2nd Ed, there were complaints that a fighter didn't need to have any strength at all since he could buy a belt of giant strength and make up for it, so we change it to a bonus instead of a set number.
3) People would stack large amounts of spells and magic items together, so we make bonus types so as to avoid over stacking of bonuses.
And the new rules they came up with were based on their new assumptions:
1) We know that the average 10th level fighter has a +3 weapon, a BAB of 10 and a strength of 22 (assuming starting at 16, getting two points by going up levels and getting a belt of giant strength +4), that gives +19 to hit. We want them to hit around 60% of the time, so we'll set the enemies AC at CR 10 to around 28. If we want an enemy to be hit more often, we lower the AC, if we want it to be hit less often, we raise it.
2) We know that PCs can fly around level 5 and therefore any written adventure will assume the PCs can find away around a pit easily around that level
3) The average fighter of level 10 has 94 hit points so it can survive 9 rounds if an enemy hits them for 10 damage a round. We can set the monster's hit points to enough that the group can kill it in 9 rounds and it will feel like a close call.
(and a bunch more)
It is the assumptions that they are talking about when they refer to rules that "talk to one another". If the magic items are removed from number 1, it decreases the fighter's chance to hit by 25% and his damage by around the same. If damage now goes down, it causes the assumption in number 3 to be wrong causing the fighter to die BEFORE the enemy(this is obviously a simplification as you actually have to find out the enemies rough chance to hit against the expected AC and factor into the equation about how much healing the fighter should expect each round, but rather deal with the complicated math, I'm just showing the concept behind it). The idea is: Change any of the assumptions, cause problems with other rules.
There weren't the assumptions (or at least, not nearly as many) in 1st and 2nd Edition. Balance was entirely left in the DMs hands rather than the rules. And there wasn't much balance in the rules themselves, so you couldn't do much to make it worse.
The problem is that 3rd Edition started with the numbers of 2nd Edition and just tried to patch the problems. It still had a lot of the problems caused by those 2nd Ed numbers.
What I think you'll see in 4th Ed is that they are starting to put a tighter reign on the numbers in order to fix the issues with 3rd. For instance:
1) Because all monsters were designed using monster levels and the equations were too variable, one creature would end up with an AC of 30 while another one with the same hit dice would end up with an AC of 14. So, we stop using an equation to figure out the numbers for monsters, we just set them to whatever we need.
2) Because of the differences in BAB amongst classes 1 class could hit every time while another one missed every time. We'll set the BABs to exactly the same.
3) Because of the variations on what magic items people bought or were offered by their DM, sometimes members of the same class at the same level would vary wildly in their to hit and damage at the exact same level. We change the equations so that the numbers work without any magic items at all levels and limit the bonuses that magic items can give so as not to vary it too much.
However, now that they've filled in a lot of the variables with static numbers or smaller amounts of variation, you'll find that all parts of the rules will talk to each other more often and with more certainty. Thus, it will be even harder to make changes to the rules without causing the house of cards to fall down.
It seems simple to change one thing and assume that it won't cause problems. And some games will not even notice the problems since they don't focus on that part of the rules. You might not notice that battles take 5 rounds to finish whereas for the normal group they take 2 rounds. And it might not matter if you fight less encounters per day and get more chances to rest. There is still, however, a ripple effect caused by a change to the wealth by level guidelines. The further away the PCs are from the charge the more of a ripple is created and the more things about the game you need to change to keep the game intact.