Completionist

A completionist, aka completist, is a person who attempts to obtain everything in a collection, thus completing it. The terms are not World of Warcraft-specific.

Profession completism
The most common meaning of "completionist" in World of Warcraft is a player who attempts to learn every recipe in a specific production-profession. Being able to do so will "differ" that player from other players with the same profession, as it is somewhat uncommon to know every recipe. In short, it is one of many ways to prove one's skill in the game. On a more practical side it may increase the player's potential when it comes to money - being able to produce items which most other players cannot will often create a market (see Auction House, trade chat) with very little competition. Also, knowing many recipes may improve a player's standing with other players; guilds will often require players with a moderate to high skillfulness in different professions to support its members.

There are several reasons why acquiring every recipe in a profession is a challenge:

Drops
A recipe may drop as loot from one or more mobs. There are many reasons why this can be a problem:
 * A recipe may have a very low drop rate, such as world drops.
 * It may drop from mobs which are too hard for a player to solo. This creates two problems:
 * Only one player can use a recipe-item. If the mob dropping it is defeated by several people with the same profession, they will have to decide who the lucky player is. Often they will use the game mechanics to do so instead through roll.
 * If the mob (see instance) is no longer popular (usually due to an expansion) it will be difficult to assemble players for the fight. This will be even more difficult if the player is somehow not eligible to enter the instance though the LFG tool.
 * If the recipe drops from a boss in a raid it may take weeks before the recipe finally drops due to raid lockout.

Reputation
A recipe may be sold by a faction which is difficult to and/or extremely time-consuming to gain rep with.
 * If rep is gathered by killing mobs, the mobs may be hard to defeat and/or difficult to access. One example is the Ashen Verdict.
 * If the rep is gathered through daily quests it will often (if not always) eventually very boring to grind it. One example is the Kalu'ak.
 * A vendor affiliated with a specific faction may be hard to access. One example is Ythyar.

Vendor
A vendor which sells the recipe may be hard to access:
 * A recipe may be sold only by vendors of the opposite faction (Horde or Alliance). This kind of recipe can still be traded through the neutral Auction House though, given the recipe is usable by members of the opposite faction. Several cooking recipes fall under this category.

Restriction
Some recipes are only available to specific classes, such as Schematic: Annihilator Holo-Gogs. While this kind of recipe can be "discarded" by the classes who cannot learn it, it will still be a recipe other players may learn (which is slightly bitter). Another - somewhat infamous - example is Recipe: Thistle Tea which after patch 1.3.0 has been BoP. Copies of the non-BoP version are extremely valuable.

Achievement
A recipe may be rewarded from completing a specific achievement which very well may be hard and/or time-consuming to complete. One example is Recipe: Seafood Magnifique Feast which is rewarded from (Guild achievement). Another example is the Wispcloak which is taught by tailoring trainers if the player has completed.

Quest
A recipe may be rewarded from a quest which is hard to complete. One example is Plans: Sulfuron Hammer rewarded from A Binding Contract which requires a Sulfuron Ingot to complete. These ingots will only (and not always) drop from Golemagg the Incinerator, a boss in the Molten Core. Another prime example is Recipe: Dirge's Kickin' Chimaerok Chops before the quest was removed from the game.

Removal from game
Recipes that for some reason has been removed from the game pose perhaps the largest threat to completionists, as these recipes are no longer available by any means (except from ranting about it on the official WoW forums). One example is Formula: Enchant Weapon - Executioner which was removed in patch 4.1.0 when Zul'Aman was revamped. Some of these recipes may later return, such as Kaldorei Spider Kabob.

Achievements (in general)
Achievements are notifications, proofs and rewards for performing different actions. Many of them are part of a category, such as PvP or Professions. Many players are very enthusiastic about completing achievement, and completing all achievements within a category is often an impressive feat. The most extreme players are going for as many achievements as possible. Three of the most "completism-like" categories are instances, exploration and guild (notice that these do also have their own sub-categories).

Instances
Instances (aka "dungeons") make up a large amount of the entertainment in World of Warcraft. As such, many players devote themselves to experience (explore, overcome, etc) the different instances. There are several achievements which also will track this progress, such as, and.

Exploration
Traveling all over World of Warcraft will take some time and reward exploration-achievements such as and. They will eventually join together, ultimately yielding Universal Explorer.

Guild
Many guild achievements focus on different types of completism, usually (if not always) signalizing, encouraging and rewarding cooperation among the guild members. Examples are and.

Key factors for success
Each recipe and achievement has its own instruction for obtaining/fulfilling. There are however some key factors which all players who could be called "completionists" should possess:
 * A decent ability to solo
 * A mediocre to large sum of money (especially important for recipe-completionists, for buying stuff on the Auction House)
 * A good social network, such as many friends or a helpful guild (for helping out, alerting, etc)
 * An ocean of time (for grinding, checking the Auction House, etc)
 * Persistence and patience (what didn't happen today may happen tomorrow - or perhaps next christmas)