Yesterday Mike Mearls put out an article addressing high level play. This is something that has concerned me since the early stages of designing Era. In the article Mike addresses a key element which is "power" and thus the power growth that leads to it. He puts a sentence that is crucial in showing that they're going in the right direction "To start with, we're moving away from a steady, linear progression of bonuses." Not only does high level play depend on this, so does every other aspect of D&D play.
Historically D&D has been caught in what I like to call the battle of the curves. Fighters move along a linear curve and magic users along an exponential one. All types of ideas have been put forward to try to "curve the curves" and make the straight line a curve or iron out the curved exponential one into a nice straight line. Seeing the futility of this exercise I decided to start from scratch an place a self regulating power measurement called XP Tax. Simply put XP Tax represents the cost of current skill and power upkeep. The more power you have the more you have to pay for upkeep. On the other hand the more power you have the easier it is to obtain that upkeep. So from the character's point of view it doesn't feel like an impossible hill to climb. The character sees it as a constant challenge rather than one that grows exponentially (A bit of tongue in cheek, those doing the math will notice hardship as XP Tax reaches 100%, but in daily gaming the statement holds).
The usage of XP Tax gives power a different curve to grow on and allows the player to retrain. Since XP Tax is a percent of your XP income it can never exceed 100%. When you reach 100% your character can no longer grow, it costs so much to upkeep that there is nothing left for the character to acquire new skills and powers with. This forces the player to play in a way that isn't always "more more more". The character can forfeit some skills by not paying their upkeep cost and make room for others. The player can keep playing by modifying the character to the new arising needs of the high level campaign. By leaving behind the skills that got the character to these levels and obtaining new skills that are more fitting for these higher levels the player can continue play, but do so without a character that is overly powerful and prone to unbalance the game.
The following graph shows four types of growth; exponential and linear in blue and red respectively. They are the classic magic user vs fighter curves. When a cost for the current skills are added the curves begin to flatten out or become asymptotic ("a line such that the distance between the curve and the line approaches zero as they tend to infinity"). While the logarithmic (log) curve isn't truly asymptotic it is so for practical game purposes (you're not going to reach level 1000). Yet when you put a percentage cost to XP then it truly becomes asymptotic and behaves like the green line. The character will never be able to reach beyond a certain level (100 % XP cost). That effectively caps game power, but added to retraining rules it simply promotes the game in a different direction.