Gaming theory: Grinding

Grinding is the terminology in gaming that's often applied to elements of tedious, repetitive tasks the player may (in some cases, often) need to accomplish in order to maintain a minimal degree of character power level or character skill competency according to the challenges they're expected to face.
Grinding does have a degree of subjectivity to it depending on the particular tastes, expectations, and how a specific player defines what is fun in a game. In my own case, for example, I often consider nearly all combat elements to be grinding unless it directly affects the outcome of a game's storyline yet I often enjoy the  logistics and industry elements of a number of games which a lot of my friends simply cannot stand.

Cons
Unnecessary padding of game duration (though many times considered a pro, especially in subscription-model Massively Multiplayer Online games (MMOs)
Lacking entertainment value (subjective)
Emphasizes and reinforces character skill over player skill (more in commentary section below)

Pros
Aids development of reflexive player patterns (quicker player response times for common strategies)
Emphasizes and reinforces character skill over player skill (again, more in commentary section)
May be used to establish a period of learning strategies, features, etcetera with repetition of new elements

Commentary
There is both good and bad that can be said about a game that focuses more on character skill. To its benefit is how it allows for a slightly more fair playing field for casual or less experienced players, something that many MMOs will need to thrive due to their reliance on a much wider player-base. It is also a heavily criticized element of gameplay, for the same reasons, by hardcore players who often feel lacking challenge from games that implement grinding systems and which essentially handicaps development and implementation of more advanced strategies.
An important thing to keep in mind about grinding is that it tends to be more rewarding when considered as a rate equation (rewards over time) rather than a work equation (rewards over effort). This means that low-end efforts tend to pay off a lot better than high-end efforts due to lower chance of failure and a steadier but less impressive income of rewards. These sorts of efforts support the practices known as item or gold farming (randomized reward game elements have a higher output with higher numbers of  weaker opposition defeated than fewer but higher quality opposition) and is sometimes why the practice of grinding is called xp farming.