One lesson that Rob shared that we've heard from numerous designers in the past is that, when a particular unit or strategy seems underpowered, you want to overcorrect it and make it more powerful than you think it should be. That way, you'll actually encourage players to try the new units and strategies and you'll have a better sense of where the true balance should be.

Rob also revealed that any discussion of balance has to take player psychology into account. The first lesson here is that changes to the game balance are almost universally perceived as a negative thing. We remember all the mail we got from the Barbarian nerf from Diablo II, so we totally get this. So rather than addressing the balance from a purely objective standpoint, it's also worth considering the overall perception of balance in the players' minds which is based more on experience than spreadsheet calculations. The original design for World of Warcraft included an experience penalty that got progressively larger the longer you played in any single session. The players felt that they were being punished for spending time with the game, so the developers simply reversed the process to give a short term XP boost to players who haven't logged in recently. In terms of the math, it's exactly the same thing but it creates an entirely different impression in the player's mind.