d20Dwarf said:
Sure, the tools change, but the baseline assumptions never do.
The system itself assumes that your character will have X,Y, and Z items to boost his stats and defenses up to a certain level so you can take on that "appropriate" monster with 20% of your pre-defined resources.
That's why you have the magic shop assumptions, because if you don't let PCs customize their loot, then they're going to be dying left and right because of the game's assumptions.
There *are* assumptions in the d20 system that affect gameplay in significant ways, always have been, always will be.
I think you've focused somewhat on the key. There need to be assumptions, and in earlier editions there were less of them, and there were some that were different. At times there were stated assumptions that were contradictory (witness the "keep magic rare" assumption, and the actual adventures which were far from that).
However, where many people fall down is they assume the core rules "assumptions" are rules. They aren't. They are guidelines they are working with. If you are creating an adventure to publish, you should probably stay within the assumptions. If you decide to stray, state that you are straying and why, so DMs can adapt your adventure easier.
Plus, there are assumptions people are adding to the rules that aren't there. I keep hearing about the assumption that you will have 13 1/2 encounters of your CR and go up a level. That's not the assumption. The assumption is
if you have 13 1/2 (or whatever the actual number is) encounters of your CR you will go up a level. It's not a rule that all encounters must be of your CR.
CR is supposed to be a
tool for the DM. If I have a 4 PC party of 5th level, I know that an encounter with a CR 2 creature should be a cakewalk for the PCs if it comes to combat (as always, barring oddities I might take into account). I know that an encounter with a CR 8 creature should probably be close to impossible for the PCs to defeat in combat. Neither means I can't have the PCs have either encounter.
Unfortunately, some people have wrapped their heads so tightly around the assumptions, that they are treating them as rules. You have the stories of players complaining because the CR of an encounter was too high. You have stories of players complaining that they don't have enough wealth for their level. There is nothing wrong with tweaking these things, as long as you understand how it affects the campign (cutting down the treasure and keeping everything else the same is going to make things incredibly difficult for PCs).