I'm proposing a section on each game where a user can post an achievement guide for the entire game. Rather than specific guides for each achievement, this would be one large section that details a route to take to knock out all achievements for the whole game. It would be useful in situations where certain achievements will naturally be unlocked on the way to another, and no special effort is needed, or if the same tactic can be used for several different achievements with minor tweaks. This might also create a way to save gamers from screwing themselves over on one achievement by taking a certain route to another - the example that comes to mind is missing an achievement that's only attainable on a high difficulty level because the player went with a lower difficulty to make other achievements easier.
RPGs are the main area where I think this would be helpful. With some RPGs having 100s of hours of gameplay, it would be brutal to have to replay the entire game to snag one minor missable achievement just because the player didn't think to check each individual achievement's description ahead of time.