I really enjoyed the idea of encounter powers in fourth edition Dungeons & Dragons. It solves a problem I've had in the past when trying to game balance characters with multiple powers (most commonly Champions characters). If you try to give a character a bunch of powers to select from, then in any given situation, there will often be one power which is clearly superior to the others. This is most noticeable when the powers are not balanced and one power really is better than the others. But even if you balance the powers so that, overall, they are equally effective, as long as the powers are meaningfully different, in any given situation one power will be better than the others. If the powers are balanced overall, then over the lifetime of the character all of the powers will be used frequently. Unfortunately, it is quite common that against a given opponent in a given battle, the same powers is always the most effective power. So the character tends to use the same power over and over again in a given battle, which is not interesting.
This isn't the entire problem, however. If a power is a conventional damage-dealing attack, using it over and over again at least works well from a game mechanics perspective. But some powers just don't work very well if you are allowed to use them without end. Entangle was a classic power of this sort. All it really did was allow you to spend an action to try to make an opponent lose some number of actions. If the opponent can break out of the entangle too easily, you wouldn't want to use it against them. But if not, after they have struggled and managed to dramatically escape from the entangle, it is very efficient to entangle them again. This is makes for exceptionally long and boring fights. It also does not match what happens in the comic books, even though from a efficiency point of view it seems like a very logical action.
What is worse, a balanced entangle power would cause the opponent to lose, on average, only slightly more than one action (after you have factored in the hit probability and so forth). After all, it is only costing you one action. But this typically isn't very much fun to use - you want your entangle be more powerful in order to “feel cool”. But if it is, you have the problem mentioned previously of wanting to use it over and over again.
I had been pondering a number of ways to solve this problem by limiting or preventing the character from using the same power repeatedly. But I must confess, I hadn't quite hit on the idea of avoiding untested and complicated solutions in favor of the simple idea of generally making powers work only once per encounter. As soon as I saw this idea, I was struck with the elegance of the concept, in terms of making a rule which, while perhaps not perfectly true to cinematic reality, would solve some problems in a straightforward fashion.
By making a power an encounter power, you have the liberty to make it powerful and exciting, without worrying about the character using it repeatedly. The power no longer has to be balanced with the other powers. It doesn't even have to be balanced with other encounter powers on the same character. You entangle the bad guy once in a display of power, he struggles and eventually gets free, and you move on to other powers.
However, in playing with encounter powers, I have noticed a problem which I did not immediately think about. They cause the characters to be "front-loaded". Encounter powers are more powerful than other powers, but you can only use them once per combat. This means that eventually you run out of encounter powers - often rather quickly. This causes some problems:
1) Traditional storytelling drama suggests that heroic adventurers should often start out barely able to hold their own against the ferocious monster attack, then rise to the challenge by “pulling out all the stops" and finishing off the monsters with a series of dramatic moves. But D&D works the opposite way - the characters pull out their best maneuvers at the beginning of the combat and peter out toward an ending anticlimax.
2) As the fight progresses, the characters become less powerful, which can make the fights more “touchy” and harder to balance. A lot of the player damage will be dealt early, then the players have to finish off the monsters with weak attacks. If the players get a little bit lucky, they may finish off the monsters with powerful early attacks rather suddenly. If the players have bad luck, they may not cause enough initial damage with their powerful attacks, and the fight could go on for a long time. Increasing the variability of fight duration is not desirable - fights with a normal duration are mostly preferable to fights which are over too quick, or which drag on interminably.
3) Since harder fights have tougher monsters which can do a better job about outlasting the initial burst of powerful attacks from the characters, it might be very hard to finish them off – while weaker opponents are likely to be killed all too quickly. Thus harder fights become harder and easier fights become easier. This is a minor issue since daily powers and adventure design can compensate, but it is not desirable.
4) Not only do the characters become weaker as the fight progresses, they also become less interesting, reinforcing the idea that a long fight will turn into a tedious slog.
5) If a fight suddenly takes a turn for the worse, it is possible to pull out your daily powers to try to turn things around. But since they are available at any time, and it is most effective to do your best maneuvers as early as possible, they are less effective when used this way then when you predict that a combat will be difficult and use the powers at the very beginning to make sure that the fight won’t turn against you. This is not a major problem, but it is kind of unfortunate that you would be rewarded for making the fight less interesting.