This is the best of all the Achievement articles this week. The author does a thorough job of considering the utility of achievements, their affects on gameplay, and how that dictates the development of achievements.
This is my same argument for why achievements shouldn't reward you for playing the game: that's what the game is there for. Instead, achievements should be rewarding you for playing well and for rising from proficiency into mastery. They extend the depth of the game by offering optional, but worthwhile challenges for the player who has already finished the (these days, simplistic) game at its core.
I do question whether it's really necessary to have an entirely separate notion as an achievement. Couldn't games implement the same sort of idea within the challenges and objectives that make up the game? Consider World of Warcraft: there are hundreds of quests, where almost no quest is required, but some make up the central narrative of the story. Why not provide a path from beginning to end that is simple enough, but provide additional paths and content that are of a more challenging level? Casual players can play through the main story and feel satisfied, while the more hardcore can explore the rest of the content, rewarding them with more content and bigger challenges. It even provides the social posturing that comes with mastery -- you can brag to the noobs about all the cool raids that you had to practise for weeks to complete and all the amazing sights, sounds, and loot to be found. Not only will they be envious, they'll be driven to become masters themselves.
I agree with the fears that achievement-whoring becomes a part of the overall game. Players know about the achievements and the cool new toys that come with completing them. Even the most casual player who just wants to play a game is going to feel ripped off that he can't "play the whole game" because he doesn't want to devote hours on end to completing achievements. There will always be those who want everything for nothing -- it's the way of our society these days. Even TF2, where the unlocked weapons aren't any more powerful than the originals, leaves people in the cold when they can't use the items until they work for them. It essentially leads to a caste system, where those with more time and determination are allowed to enjoy more of the game.
There are ways to deal with the "achievement-whore" problem. Several people have made achievement maps for TF2. While these may be frowned upon by some (it's where all the achievement-whores do their thing), the truth is that it allows people to work strictly on achievements, keeping them from interrupting those playing the core game. Without achievement maps, those same people who really want to get achievements would be in the main servers, chasing down achievements instead of going after the map objectives.
I wholly appreciate the example of locked characters/tracks/weapons in racing and fighting games as bad examples of achievements. There's nothing more insulting after spending $60 on a game to be told that you have to work before you can play the game that you just purchased. It also makes game reviews less reliable. Inevitably, reviewers don't have time to unlock everything and play beginning to end. The result is two options: play a small subset of the game with only a few characters and maps, or use a code to unlock everything and miss the experience of having to unlock everything legitimately. It creates two mutually-exclusively paths, resulting in only half of a thorough review. We see similar problems with reviews of MMOs: the first 10 levels are fun, but everything after that is repetitive and boring. But reviewers don't get a chance to make it past the first 10 levels, so they give glowing reviews, whereas consumers throw the game out after a couple weeks of playing it.