As Odit said, I really like how CoD 4 do their achivement, since it's so objective(don't ruin the match) and it progress smoothly (count how much kill you got with specific weapons). Basically it emphasize new play style for every player to try out, after playing that style you and earned achivements, you unlock new tools which specialize you more into that style of play. This is a well planned achivements system really.
Don't amke it for a stunt play like TF2. TF2 achivement, at first i think they just make those without gameplay effect reward(for awhile it is). But later they added new weapons reward for achivements, that's when ppl start farming achivements and achivement map get more attentions.
The best implementation of achivement, i give it to Guildwars. GW got titles which do nothing just for a show-off and they got great looking armor (harder to get) which is not different in stats from a cheap, poor-looking ones. So everyone can fight in equal ground, new player with right determination and adequate knowledge of skills can kill the veteran who slack off or make mistakes.
Well in multiplayer there is going to be options to play games in a "persistent" world so you carry over achievements to other games, but these are of course are not ranked, so there is nothing wrong with doing that. Ranked games are going to be fair and its not a matter of who has played more and got more achievements to decide the winner.
It actually sounds quite fun to every now and then take a break from serious matches and try leveling in the persisten wolrd
Is this true? if it is that'd be so perfect, seperated world of Serious PVP and Casual plays to satisfy both needs.