Power Sources

Previously, I went through a bit about what a Power Up is.  This time, I will discuss ways in which Power Ups in all their success promoting, gameplay modifying forms are actually seen in the game.  Here are some categories that I have come up with, keeping in mind that some Power Ups can fit  into multiple categories:

Utility

These are the type that do not modify the gameplay directly, but still help the player towards success in the game.  These do things like replenish health and ammo, permanently improving capacity, unlocking new passageways, money, experience, etc.  It isn't that these power ups can't change the gameplay, it's more that the change it has on gameplay per individual item is insignificant and the change in gameplay is psychological on the player's part.  Someone with higher health capacity and ammo is more likely to take more risks and be more liberal with their ammo, but the gameplay itself hasn't changed, just the way the player feels about it.  And even though these items don't effect the gameplay inherently, the mere inclusion of a power up will always effect gameplay (typically lowering the difficulty as well).  Things like coins in Mario have long been used to draw players towards the correct path, even though their collection does not modify gameplay, and even their individual utility to the player is fairly insignificant.  Healing items in RPGs are another interesting one, because proper management of them can be a fairly integral part of the gameplay.  These types of power ups are nearly ubiquitous in gaming, and it is generally harder to create a game without implementing these in some way.  The regenerating health that has become a staple in modern FPS games is actually a really good example of this.  Whether a design like this is an improvement over health lying scattered around is something that varies greatly.  The value of these Power Ups is mostly dependent on their rarity.  Extra lives can be a great reward to skilled risk takers in a game, but if they are common enough that the player easily hits the maximum, there might as well not be lives at all. 

Temporary

These are Power Ups whose effect is temporary.  Whether a time limit, damage, death, running out of ammo, or being single use, these will run out.  These are the primary type of Power Up found in platformers, and to a lesser extent includes the weapons found in many FPS games and classic shoot em ups (death and a new stage generally resetting the weapons and abilities the player has accumulated).  Essentially anything that does not persist can be seen as part of this category.  These are most commonly seen in linear games (meaning the game is broken down into stages, and the goal is to get from point A to B, however circuitous that path may be) though they do show up elsewhere.  This type actually rewards player skill, since these power ups can be easy to find but can only be fully utilized by proper skill from the player (getting extra lives from an invincibility star, maintaining a fire flower the entire game, saving the rocket launcher for a later tank instead of wasting it immediately on infantry).  Their temporal nature is also valuable because they can add variety without changing the baseline experience, but the game has to be designed to be completable without these temporary Power Ups, with only a few exceptions.  A rare example of this is the Hypermode ability in Metroid Prime 3 (you may choose when you enter it, but it's still temporary) turns on for the remainder of the game once the final area has been entered, which causes a permanent shift in gameplay.  This sort of design is definitely more the exception than the rule, both in its forced use at the end of the game, and in its ability to be chosen to be entered at any time.

Granted

These are permanent Power Ups that are given to the player during the course of gameplay.  Unlike the previous category, these usually come after some sort of accomplishment in the game like finishing a stage, defeating a boss, or finding some obscure and difficult to reach corner of the game world.  Because these are permanent, they effectively change the baseline experience for the game, which might make one wonder why this would be superior to something temporary or having all of the capabilities immediately.  Besides the obvious at this point (variety) there is also the difficulty curve of the game.  Granting abilities after the player has overcome a challenge that proves their proficiency with the current gameplay mechanics allows them to learn incrementally.  This is a much more effective system than a tutorial info dump of everything at the beginning of the game.  At the same time, there are some serious balance complications with this, as granting a large amount of Power Ups over the course of a game can cause many of them to be used once and forgotten about.  Also if granted to close to the end of the game, it ends up having very little actual application, which can mean a great deal of time implementing a Power Up with little benefit to the whole experience.  Portal 1 is an excellent example of its use, as there are only 2 Power Ups:  Creating a portal, and Creating a second one.  You don't get each one until you've managed a number of different situations first that proves your mastery, but the majority of the game is spent utilizing those two Power Ups effectively. 

Chosen

These are also permanent (with few exceptions) but instead of being received at given points in the game are chosen by the player, typically by means of some list on a menu and spending some form of points.  This gives the player gives some options in terms of strategy and play style that is not available otherwise.  However, non-determinism of any sort can be very hard to design around.  Whether intentional or not, there is often an optimal order, and some other combinations could be severely worse.  Designing the game difficulty to follow the least optimal order could leave it completely unchallenging to better set ups, and designing for the most optimal could make it downright impossible to worse set ups.  Which means the best solution is to plan for something in the middle, which may still be a far from either extreme.  The ideal solution would be to balance the possibilities well enough that there is no optimal order (or at least very subtle differences) but that is difficult without greatly removing the actual variety of it.  These sorts of Power Ups are common in the form of research upgrades in Real Time Strategy games.  In this case the player starts from scratch at the beginning of each level, so an optimal order for acquiring them can simply be seen as part of the overall strategy, and a poor decision will only effect the immediate level.  In a hack and slash (another place these have become common), the cumulative choices since the beginning of the game could have consequences much further out.

Slotted

These are Power Ups that are selected from a pool of possibilities, with a finite number being usable at any one time.  This has become popular in many multiplayer games lately due to the larger meta game strategy that it allows.  This of course has many balance issues, as some combinations can be overpowered to the exclusion of all other strategies.  If the number of slots and size of the pool can be increased, balance gets that much harder, and individual Power Ups can't be very significant due to the differing quantity.  This type rarely stands alone, as slots and Power Ups have to be acquired somehow. 

Chained

These are Power Ups that have others as a prerequisite.  This type can not really stand on its own, but is a combination with others (Mushrooms before Flowers, Skill Trees, etc).  This allows for connecting different types of Power Ups into very complex formations. The prerequisite is in the actual possession of the Power Up, not in the gameplay side effects.  This is most often used with Slotted, allowing certain Power Up effects to get progressively more complex, but Chosen is often included as well.  This can keep player choices from completely unbalancing the game. 

Compound

These are Power Ups that are useless individually, taking effect after a group have been collected.  This is another type that does not really stand alone, with Utility being the most common combination.  There is absolutely nothing preventing other combinations though. 

In Practice

Naturally, there isn't a game that uses any of these exclusively, even if you only consider Chained and Compound an extension of these other types.  But this helps shows some of the concerns that different implementations of Power Ups can have.  Some of the issues come from choosing one type where another type would be better, others come from just not considering thoroughly the complexities of a certain implementation.  And of course, the more complex the implementation gets, the more to consider.  Leveling systems are a specific combination of these more basic types of Power Ups, and I'll get into more the issues with those later.