Creating balanced, playable mechanics for a game system is the overriding reason for the power framework. It's like you're asking 'why do objects fall, besides gravity?'. I could make something up, but it would be BS.
There are two parts to that statement: balanced, and
playable.
Lets create a new power:
"Leap of the Heavens"
Encounter
You spring out in an astonishing leap. Shift yourself four squares in any direction. You may clear obstacles up to four squares (20') high, and may change your vertical position upwards or downwards by up to four squares.
What about that power means that it should be an encounter power?
I could live with this:
Keyword: Ki (although, that doesn't exist anymore!)
Explanation: You focus your Ki into a tremendous burst of energy.
This builds on an assumption that your Ki is a limited resource which generated on a timescale longer than a typical encounter.
(Although, with that explanation, I would prefer Ki powers to have the limitation: You may freely use your Ki encounter powers, however, if you attempt to use more Ki powers than your power limit -- the number of powers known -- you must make a Ki power check or be stunned for one round. A Ki power check is a level check against 10 + the Power Level, where the Power Level is either the first level at which the power is available, or your current level, for powers that scale with level.)
What I'm looking for is some consistent explanation for why the power works the way that it does. For Rage, the explanation is that you are too tired to rage again. For many skill tricks, these are only usable once per target per encounter, and the explanation is that the trick only works when the target is not guarding against the trick. For many abilities, such as fear effects, once you save against the fear effect, it won't work against you for a day, with an explanation that you have learned to see past the illusion that cowed you before.
Let's look at a different example:
Vampire's Gaze: Standard
Effect: One target
Result: On a successful attack vs will, your target is dazed for one round and their next action must be to move towards the vampire. If already adjacent to the vampire, the target may not move away from the vampire. The target may not attack the vampire until this power expires.
Vampire's Kiss: Standard, at will, target must be under the effect of Vampire's gaze.
Effect: One target adjacent to the vampire.
Result: The target loses a healing surge. If the target has no healing surges, they loose 1/4 of their hit points. See Unholy Puppeteer. A successful vampire's kiss recharges Vampire's Gaze.
Unholy Puppeteer: Standard, at will, target must be subject to Vampire's Embrace with no healing surges. Refresh: Move, Attack vs. will.
Effect: One target, for one round.
Result: The vampire takes control of the target creature, and may choose a single move and a single basic attack for that creature's next actions.
Explanations:
Vampire's Gaze is a magical effect that is continuous and inherent.
Vampire's kiss is innate to the vampire, but requires a receptive victim.
Vampire's kiss weakens the target, eventually leaving them helpless to the vampire's demands.
In this example, why chose the Vampire's Gaze to be at will? My presentation is that this is a balancing between how one would imagine such an effect would work, and what works as a mechanic.
Both sides of the scale must resonate with the player and DM. If only the mechanics and balance were considered, one is left with a meaningless (but possibly interesting) game of abstractions. (Why does a horse move in an L shape?) On the other hand, it's not just enough to have a mechanic that seems to fit a particular creature, it has to still have an effective implementation. In the case of Vampire's gaze, the the effect (a visual domination) is iconic, and fits my sensibilities as a continuous ability ("My companion cried, 'For the love of all the saints, don't look into his eyes!").
Now let's turn this into a Player ability:
Gift of the Vampire: Bloodline: Encounter
Your ancestry asserts itself, granting you a measure of the power of a vampire. Your ability is weaker than that of a true vampire, while retaining a measure of potency.
Effect: You may use Vampire's Gaze once per encounter.
Why is that a once per-encounter ability?
1) True vampire's can use it at will, basically, once per round.
2) Your ability is weaker than that of a true vampire. You must conserve your power to achieve the effect of a true vampire, and you are quickly exhausted.
Then:
Blood Infusion: Bloodline: At will
Prerequisite: Gift of the Vampire
You have infused yourself repeatedly with the blood of a true vampire, and are now able to maintain the potency of your bloodline abilities.
Effect: You may now use Vampire's Gaze at will.
Explanation:
1) You are now more infused with blood powers. These regenerate quickly, in the manner of a true vampire, such that your Vampire's Gaze effect no longer exhausts your power.
That seems close to a usable power. So far, two issues have been covered: First, the power has a resonant explanation. Second, the power has an implementation.
However, I don't know if that is a balanced power. (I suspect that it is a bit overpowered.) That leads to questions: How well does the implementation work? Is it too complicated? Too boring? Too under or overpowered? Does it break any game assumptions? (For example, save or die is forbidden; taking away control is frowned upon.) Those would have to be answered to put this into use.
The point of all of this is that
both the issues of the game mechanic, and issues of the background flavor of the power, must be brought into the picture when adjusting the power. (I
think that folks objections to 4E powers is that the game takes away too much of the discussions of the background flavor, leaving too much stale abstraction.)
Notice that this not just "fluff". There is a logic and sense of meaning in the background flavor that is being used to decide what makes sense for the power.
(As another example, consider the power:
Ten-thumbs: Weapon, Encounter, At-will
Effect: Strength vs Strength, or Dexterity vs. Dexterity.
Result: Your opponent must use a move action before using their weapon or implement.
I'm using that to abstract out a "disarm" ability.
Does it matter if I use "Disarm" as the power title, instead of "Ten-thumbs"? Which title is more satisfying? Are you satisfied with an ability presented as just the short three line description as was provided, or do you hunger for more that that?)
Thx!
TomB