Since Matt already beat me out for the ceremonial "FIRST!!!!!" post (he posted "test," but we all know what he was thinking on the inside), I suppose I'll have to discuss something of substance.
Achievement in games is often treated as only some signifier of the fun you had. Unlocking achievements in a racing game just means you raced a lot. Unlocking achievements in a FPS just means you "pwned" a lot of "noobz." The achievement itself means nothing, when you sit down and play it carries no weight, your performance and enjoyment of the next play session is no different with or without the achievement.
The exception to this is the RPG. Your achievements are not just part of the game, they define the game through experience points, levels and quests. The "kill 10 rats" quest is not some side feature of a game about killing rats, killing rats is almost a side activity in a game about completing quests and gaining levels.
While a lot of discussion is put into making the process of killing rats fun, very little discussion is put into creating the creating the best sense of achievement. This is in spite of the general acknowledgment that in RPGs (and more specifically MMORPGs) players will do whatever lets them reach their goals fastest, even if it is significantly less fun than something else they could do.
In short, when given the choice between achievement and fun, players usually choose achievement.
The interesting thing is that the response from developers is still to focus on the fun. It isn't an attempt to make the best achievements, it is an attempt to use achievements to force you to have their fun. Developers try to make it so that the most fun thing to do IS the fastest way to reach the goal. When players find an unintended faster way to reach a goal, that new way is labeled an "exploit" and treated as a huge problem because it circumvents the intended "fun" way.
To me, this is the most overlooked aspect to an RPG, especially an MMORPG. I feel that delivering a good sense of accomplishment to a player through proper challenges and appropriate rewards is more important to the "addictiveness" of an RPG than the fun itself. It would seem that the online gaming community agrees, at least when voting with their dollars. The original Everquest had a reputation amongst its own player base as being "not fun," yet they continued to play, and that game was king among online games for several years. On the other hand, City of Heroes was by far the most fun online game I have played, yet it did not ever enjoy the success of even several other games aside from WoW.