Now, to be fair, Psychonauts gets half of its collection sidequest stuff right. The game is divided between the 'real' world and the mental world, and in the real world, collection works like this: Collectible items (cards, challenge markers, scavenger hunt items, and brains) are brightly colored and mobile, making them pop against the game's backgrounds. You're given a counter in each area, telling you how many many of each collectible are left. And, most importantly, each collectible has a reasonable amount of worth. Every collectible is worth either 1, 1/2, or 1/9th of one of 101 Psi-Ranks (the game's leveling-up aspect, which is completely based on collection), and the challenges associated with each collectible are proportional to the value. A Psi-card (1/9th of a rank) is easy to grab, while a Psi Challenge Marker (1 whole rank) will involve some serious searching and acrobatics.
Compare Psychonauts to Mario 64. Figments are worth a variable amount of a percentage of a Psi-Rank (usually proportional to how well they're hidden), and discovering them all unlocks a hidden ending cutscene. Which is another way of saying 'There's part of the game's story that you don't get to see if you don't get every figment' (barring Youtube, of course). In Mario, the basic collectible unit is the coin. Most levels have a total of between 110 and 140 coins, and the player is rewarded with a star for collecting 100 in a single run. After that 100, the only benefit to collecting coins is health restoration. That makes the 100-coin star challenging, but not maddeningly frustrating. It's a challenge to your collection abilities, but doesn't force unpleasant, completionist behavior on the player.
The irony is, if Psychonauts DIDN'T penalize/reward you with additional story content for collecting every figment, it would make the act of completing the task (which the game never explicitly asks you to do, but which is implicitly demanded by the ____ out of _____ figment count for every mental level) even more hollow. How many games have you played where, upon reaching 100% completion, you get, at best, a text box, and then nothing else? Once you've set a goal (by 'telling' players to get EVERY figment), there must be a reward, or the player feels cheated. (All of this presumes that gameplay like this can't be its own reward, which is easy to say, given how stressful, nerve-wracking, and boring scouring a level for a single missing figment can be. At this point, you're essentially being bribed by the game into playing it in a particular way. Bad design, I think, especially when contrasted to the elegant way Mario 64 handles it). The game has asked you to do something that most players would consider unpleasant, so it has to use the carrot of unlocked content to guide you forward.
How to fix it? Glad you asked! First, there's the Mario solution: only a subset of collectibles are needed. This is probably the most pleasant way to go around this, because you can tune and playtest to find the right percentage to avoid needless player frustration. You could also go the "collectible finder" route - Saints Row 3 does this, and it works fairly well. This can go too far, of course - Far Cry 3 is overly insistent about notifying you of where collectibles are, turning the game from one of free will into one of watching the dot that represents your character as it moves across the mini-map, creeping toward a collectible icon. It's better than leaving players to waste hours in the unpleasant act of searching, but playing as a dot making his rounds on a mini-map just isn't as fun as playing as a character exploring their environment. It takes exploration, which is ostensibly what all this collecting nonsense is supposed to be in service of, and makes it safe, boring, and rote.