If you atomise (i.e. split it up into its smallest parts) any skill system you can see it is essentially comprised of the following raw materials:
An effect (or a number of effects). These could be damage, healing, reduced movement speed, increased attack speed or other fancy things such as teleporting the player etc. The effect can vary in three respects: magnitude, condition and duration.
A cost. Usually energy/mana/magicka. It is the resource you deplete when you use the skill.
Limiting factors. These are cast time, recharge time and other fancier things like requiring skill chaining or environmental factors.
Part of the job of balancing a skill is finding an equilibrium between these three elements; and making sure that for each point of energy/adrenaline you spend (when factoring in the limiting factors) you gain the same amount of effect for every skill in the game.
E.g. if one skill costs 10 energy and does 100 damage with a 1 second cast time and 5 second recharge, and another skill costs 20 energy and does 150 damage with the same limiting factors you could say that the two skills are not balanced, because for each energy point you spend on the first skill you get 10 damage, but for each point spent on the second skill you only get 7.5 damage.
Part of what makes balancing MMOs so difficult is that you have to factor in all these elements in relation to each other both within the skill itself and in relation to every other skill in the game (this is particularly true with GW1, considering the secondary profession system gives you access to every skill in the game) and also take into account how the effect of the skill interacts with other skills available to the player. Anyway, I’m getting way off topic, for my views on balancing head here: CLICKY!
Back to my point; learning the relative weights of each of these elements can help you look at a skill and assess its worth. You can come to an axe attack which does 100 damage and know that it is garbage because of its 20 energy cost. Or look at a fireball spell which causes 50 damage for only 5 energy and know that it is essentially useless because of its 45 second recharge. On the flip-side, you could spot that a skill which does 35 damage and know that it is the dogs-bollocks because of its 1 second recharge time and ¼ second cast time.
Effective team builders are able to do this process of weighing up a skill’s worth in the context of the bar itself, in relation to every other bar in their team’s build and also in relation to the enemies they are likely to face when they step into the combat zone. The vast majority of you do these kind of mental gymnastics almost every time you log into the game, it probably comes naturally and you don’t even realise you’re doing it.
It becomes more impressive when you use it in the field: it’s more than just reacting to a gunshot in Call of Duty or deftly taking the ball around a keeper in FIFA. It is lightning-fast mental arithmetic on a grand scale; if an Iboga casts Conjure Phantasm on your Elementalist and you see that he is struggling to stay alive, you quickly calculate the advantages and disadvantages of using the mechanics and tools set in front of you. If you use your hex removal (15e, 1s cast, 15s recharge) it will remove the threat initially, but the Iboga will likely recast his hexes pretty quickly and you will have spent 15 energy and be back to square 1 without a hex removal, so that’s out of the question. You could throw Healing Breeze on him and hope to counteract the hex, but you know that you would just stem the tide (because you can see by how fast his bar is dropping that your regeneration will only neutralise the degeneration on your ally and not heal him). You decide on straight out healing and cast Orison of Healing on him to bring his health back up – this gives you a buffer, and will counter both the degeneration and the base damage he is likely receiving from other sources.
In the split second which you made this decision you calculated the relative weights of each of the skills on your bar in relation to the context of the situation and in relation to the relative weights of the skills on the opponents bar.
And they say playing video games rots your brain.