Imagine a scenario of a group playing 3.5e D&D when swift actions were part of the game. The group is all 5th level, about to hit 6th, and is made up of a Bard, Barbarian, Fighter, Monk, and Wizard. It is an urban campaign, so almost all of the opponents are classed-NPCs. The GM really enjoys building effective NPCs to challenge the party with. The GM wants to add a new power to the game called Heroic Concentration. It is a feat that lets you spend a swift action to gain +10 attack and defense (AC and all saves) for one round unlimited times per day. He is trying to think about what requirements this feat should have.
Heroic Concentration is obviously insanely powerful. Think about the impact to the game under the following three scenarios:
- Heroic Concentration has no requirements. Since everyone is about to hit 6th level, everyone will take it. Likely, every NPC will have it as well, so the feat is really a wash; we just increased all numbers by 10. People are probably going to be mostly annoyed. They wanted to spend their 6th level feat on something neat and now they have to spend it on this dumb power just to keep up with everyone else. If anyone relied on their swift action, they actually lost a little power because they'll always have to spend their swift on Heroic Concentration. The introduction of Heroic Concentration doesn't really imbalance anything, it is mostly annoying.
- Heroic Concentration requires 1 level of monk. Now everyone will feel like they need to take monk as their 6th level and spend their feat on Heroic Concentration to keep up. Except the monk, of course. He is happy as can be. The fighter probably doesn't have a big problem with this, but barbarian and bard can't take monk without losing class powers. Wizard also has a tough choice because the potency of the wizard class relies on getting deep into the levels. Having to take a level in monk just delays access to all the spells he was looking forward to. This is a real problem for the game. Sure, everyone can take Heroic Concentration, but the access costs are not equal for all players and since it is so powerful everyone feels like they must take it, this imbalance is a problem.
- Heroic Concentration requires 5 levels of monk. The game is broken. Even if everyone starts heading towards Heroic Concentration right now, they have to spend the next 5 levels drastically inferior to the monk and all NPCs they encounter. For the bard and barbarian, they'll spend those 5 levels also without their existing powers. For the wizard, his powers will never scale to keep up. This game is broken and the GM likely just ended his campaign by introducing Heroic Concentration with these requirements.
People might be able to disagree here or there, but the gist of my predictions for each of the three scenarios is probably accurate. Heroic Concentration was identical in all three scenarios, but the impact it had on game balance was entirely correlated to how difficult it was to access.
Think about that for a second. I mean really think about it. How often have we seen arguments over tweaking some rule because it was too powerful or too weak or too whatever? Absolutely, some things can be too powerful/weak and tweaking them can fix the problem. But more important than any of that is access. We can select different modes of access and that will have a huge impact on our ability to add rules to the game. Since adding rules is pretty important, we should think carefully about what mode of access we select. Now also think about what modes of access 3e and 4e took. Fourth edition took a pretty narrow mode of access with every class being linear and limited multiclassing. Third edition was much broader with more reliance on feats and more flexible multiclassing (albeit still linear within the class). Think which system was more enjoyable to design new stuff for--most people would say 3e and that was because it had a more accessible mode of access.
Runeward sought to address all these issues and smooth out the power curve. Through proficiencies, every class is progressing towards the high level powers of every other class, albeit at a slower pace. If new powers are introduced, they are substantially more accessible to all characters. As a result, the risk of imbalancing the game by introducing a poorly designed power diminishes. That is great design for a living RPG that people are going to fiddle with and it is something that D&D has been moving away from.