As promised, here are my thoughts on achievement systems, specifically the World of Warcraft one, but most of the comments would apply to features like Warhammer Online's Tome of Knowledge as well. Of course the subject has already been discussed on other blogs, but I'm not trying to simply praise achievements or condemn them as useless, but rather to look at what their function is.
I classify achievements roughly into three categories: The first category is achievements which just pop up while you do what you would have done anyway. For example there is simply no way to maximize your fishing skill in WoW without getting a bunch of "caught so and so many fish" achievements. The second category is about going that extra mile, and for me those are the most fun achievements. If you are an explorer, you probably already visited most of the places of the world map, so why not collect the few remaining ones and get a nice "the explorer" title? Or you're questing in a certain zone anyway, why not make sure you do *all* the quests and get the achievement that you finished that zone? The third category is the extreme one, where you are asked either to do a huge amount of something, or do something in a particular stupid way on purpose. You would need to spend a large amount of time to get 40 reputations to exalted, or to get the loremaster title for doing nearly all the quests in the game with a single character. And many of the heroic dungeon achievements are just silly, like asking you to make your life harder at Kel'thuzad by pulling a bunch of extra undead. In WAR there are a bunch of particularly counterproductive achievements like that, asking you to do various dangerous things while being naked. Of course that can get annoying if one player decides to do battlegrounds naked and by that contributes to the other players losing the battle.
So what could the purpose of all these achievements be? The first category, the automatic achievements, are just a gimmick, the game giving you a pat on the back and telling you "well done", although you didn't do anything special. The second category encourages players to use the content already provided to a greater extent. You're more likely to do something if you have a "shopping list" of things to do, and a title or pet as reward. The third category extends that to the point where the achievement is faking additional content. A heroic fight with the specific purpose of reaching the achievement for it will be playing somewhat differently than the standard best strategy to beat that boss. Thus without adding a new boss, the devs fake a new boss encounter.
Not everyone loves achievement systems. But in the case of WoW I can't really find anything very negative about the system. If you don't like it, you are completely safe if you just ignore it. All the rewards are just fluff, like titles or non-combat pets, there are no achievements where everybody would feel he would have to do those to stay competitive. For those who like achievements a little or a lot, the system provides a guideline, a list of proposals for activities you could do in the game when you have nothing else to do. So to some extent achievement systems are designed to overcome boredom and burn-out. Of course the interest of Blizzard in that is that achievement systems are cheap to implement and if they encourage some players to not quit the game quite as early as they would have otherwise done, the payout can be pretty good.
I like achievement systems with invisible achievements, like the WAR one, a lot less. You end up having to look up how to get achievements on some third-party internet site. But on the other hand the WAR achievement system has a huge advantage over the WoW achievement system in that it was in the game from the start. Adding an achievement system to a 4 year old game has some disadvantages. *I* know that I've done all the dungeons of the old world and the Burning Crusade, or that I raided Molten Core, BWL, ZG, AQ, and several TBC raid dungeons. But WoW didn't remember that. So somebody who killed Ragnaros at level 60 when it was relatively hard does not have the achievement for it, while somebody who does it in a silly raid at level 80 does get the achievement. And I don't know how much of the current instance server load is caused by level 80 players soloing low-level dungeons to rack up achievements that really don't mean anything.
So, in summary, achievements can be fun if you take them as simple proposals for what you could do in the game. They can turn into a grind if you get caught in a "must do them all" mind set. And some achievements that can only be reached in a group have the risk of some player who is trying to get the achievement inconveniencing the other players who just want to do the normal content. Ultimately achievement systems are just a cheap way to simulate additional content. As long as that works for some people and doesn't hurt the others, why not?