I used to care a lot about achievements. I used to want to 100% every game I played. That lasted about 2 months into their existence after I found one that required you to be #1 in the world. Obviously that was never happening, so i had an unfillable gap which allowed me to freewheel away from the concept.

I kinda like them, and if I'm really enjoying a game but have already beat it, I'll go for the achievements as a way to squeeze a little more juice from it, but i no longer care. If they stopped existing right now I bet it would take about 4 months before I even noticed.

This is the first I'm hearing about retroachievements but I like the concept. It'd probably push me to actually try with old games. My normal pattern now is to download a game i used to love as a kid, bask in nostalgia for about 20 minutes then get bored or frustrated with the difficulty and move on.