At the time I took the screen shot there were 2 I hadn't gotten. Complete the game and finish act 3 without dying. That's what it looks like when didn't get the achievement yet. The top left one is for finishing act 2 and the top 3 on the right are for discovering something that's required to progress in the game anyway. There would be no way to get to act 3 without first getting the achievement for either of those.

Looks to me like the gold ones are the harder ones to get or something, since the highlighted one is "pass act 3 without dying". That's something you can't reload and try again, like finish X puzzle in Y minutes. I guess it's similar to the different point amounts in Xbox achievements, or the bronze/silver/gold trophies for Playstation.

-Oh, I just realized you said you didn't have that one. I thought the gold one next to it was that, did you get this one, and was it gold as well?