The Psychonauts achievement list is an exciting mix of things you gain through normal play, pointers to little gems, stretch goals for achievers and completionists, and “dude, seriously?” The normal play ones are what you might expect: beat each level, level up, complete the game. The completionist goals are exactly what you’d expect, although actually completing everything turns into “dude, seriously?”
Pointing out little gems is one of the best things about having achievements in games. It gives developers a way to encourage and trivially reward players for seeing the best that they have to offer. “I’m Sure She’s Over It” is a wonderfully compact bit of storytelling, and it explains what might otherwise be a BLAM in a later level. “Self Aware” directs you to some subtle content you might not otherwise try, and it includes some foreshadowing for the aware player. The achievements also include some direction on what to do next if you are stuck and a few things you may not have realized you could do (like “I Think They Were Impressed” and “Stump Speech”).
Some of the achievements are unfortunate, or perhaps just unfortunate in how they interact with the game design. I still cite WoW’s The Green Hills of Stranglethorn as the worst achievement ever. It presumably was meant as a reward for completing some arduous content, but its effect is (was pre-Cataclysm?) to encourage players to slog through some of the worst content in the game. Don’t encourage your players to punch themselves in the face, and don’t be surprised when they complain about your game after they react to the incentives you gave them. For Psychonauts, those achievements are the ones that become unachievable at a later point in the game and 100% completion, which is painful here. Out of 35 achievements, 6 become unavailable unless you did them at the right time (or saved immediately before) and another 6 demand a separate “point of no return” save game; there is no post-victory wandering around the map and re-visiting levels. These stack on each other: the most painful are gathering all the figments (which are 2-D, translucent, sometimes mobile, and can be half-inside objects or entirely inside them while moving) and the resulting max level of 101 that calls for several “gather all”s. This is more of a criticism of the difficulty of 100% completion in this game, but there is an achievement encouraging you to go for it.
The percent who have completed each achievement is driven down by the late addition of achievements, although I am suspicious that even 0.5% of players have legitimately gotten a 100% completion. Contrarily, there is the Torchlight achievement page, which says that 11.8% of people who own Torchlight on Steam have presumably never installed the game.