Leveling Systems

When a game is said to have RPG elements, 9 times out of 10 that means the game has some form of a leveling system.  Using your weapon enough gives you rewards for using that weapon, killing a certain number of guys unlocks new abilities or skills, or your health increases the more punishment you take.  Leveling systems come in a wide variety of forms and are ultimately a reward system to help feed that "just one more" feeling that most games have.  You know something exciting should happen when that progress bar fills up fully.

Some games level you up at key points based on moving the story forward.  This is great for the game designer because they know exactly how much power a player has at any given point in the game, which makes balancing easier.  However, this is no fun for the player because it's incredibly predictable and does not reward the player for any skill they have or exploring they do, thus making the levels feel like part of the story rather than something the player earned.

Some games level up individual skills the more they are used.  Depending on how opportunities to use the skill are given determines how hard it is to balance systems.  If those opportunities are limited, then the game designer can calculate a player's minimum and maximum strength, but if opportunities are randomly generated and reoccur indefinitely, then a player can grind at one point and reach the highest levels early on in the game story-wise.  This system is good for the player since it makes changing play styles possible (with some grinding) and makes it very clear what a player needs to do to be rewarded.  However, players don't like to grind, so if it feels like they need to, which it usually does with this system, they will look for exploits to level up a skill super quickly.

Some games level up all the player's stats at once by rewarding a player for various things during the game (exploring sections of the map, unlocking a chest, killing enemies, etc.).  This can make balancing a game very difficult since you have to account for min-maxing players who do everything to get as much experience as possible and for casual players who just go through the story.  However, as long as it's consistent and clear what the rewards are for, this can be the most enjoyable for players.  Players don't have to worry about playing a certain way to level up the skills they want, but get rewarded for the choices they make and actions they take.

One thing all these systems have in common is they all have progress bars that fill up.  In my experience, all games except tactics games, the amount needed to level up goes up exponentially as the levels go up.  Tactics games generally give a player a level every time they get 100 experience points (XP).  The amount of XP you get for attacks/kills is based on the level of who you are attacking.  Personally, I have never understood the exponential system because gaining levels becomes increasingly rare making your character's growth more and more stagnant.  Instead with the tactics XP system, your character is constantly growing, you never get stagnant, and you can be rewarded for challenging yourself and completing tasks that are supposedly too hard for your current level.  You also never have to guess when the next level is occurring.

I'm a big sucker for leveling systems and the more there are in the game, the more addicted I will get, but done poorly they can turn something fun and rewarding into a chore that needs to be done, thus turning a game into work.  There should always be a balance between plentiful small rewards for a player and infrequent large rewards that players can work towards.