I'll have to side with Daqin on this one (LOL)...
The first thing that comes to my mind is: THIS IS NOT A CONSOLE GAME. Seriously - all unlockables, medals, badges, achievements, etc tend to annoy me as a perfectionist and as a player.
* It's like Pokemon - you want to have them ALL... which leads to hours upon hours of playing mindlessly just to add another achievement to the list. You don't have fun playing a normal game, you limit yourself to ridiculous rules and end up confused having reached another goal (you got the achievement you started the game for... Might as well quit, huh ?) - like "taking 300 HPS worth of damage in lava or toxic pools in one game".
* What FOR ? If achievements were for useful things to do in a game - you would get them anyway and the "fun" would be over too soon. Namely - after a few games. Guess what - that's why they're usually:
-> for absurd and/or suicidal behaviour
-> for taking a specific approach and taking something to the extreme
Both kinds force you to make separate games just to get them. Short and/or tedious games with a purpose so narrowed that you hardly can do anything else.
* Keeping you from accessing all of the content... That's just mean, most of the time.
* What worst about all of this ? Losing that progress in any way - usually with rest of the data from the hard drive but it's not the only way... Also - inability to take the progress with you or the irritating file swapping or the imperative of keeping the game on a pendrive.
Yeah - three things come to mind in analogy. Game modes, the highscore and the player profile.
The game modes are different because they just change the rules of the game without adding any goals. They narrow your options but not as much as a single achievement would.
The highscore is just a way of viewing what you've done. It does not require anything of you. Neither does it have an upper limit. If you want to do better - you're the only person to tell you HOW much better do you want to play.
The profile is what bothers me the most... Yet - while it's kept simple enough and the titles are as linear and make as much sense as as they do now - it's just a matter of playing normal games until you've learnt and did enough.