I'm a sucker for collecting crap in videogames. The universe could be perishing in fire and blood but if I haven't found all of the hidden dentures scattered around the current level then by God the universe can just die without me. Collecting things is one of the great dichotomies in contemporary videogames: you are either its petulant slave or you have no idea why anyone cares about such a stupid feature.
I'd like to make this stupid, indispensable feature better.
For starters, let's hit the obvious problem: what's the point? This varies from game to game. My current collection fetish object, Assassin's Creed 2, places little treasure chests all over the level and each time you find one you get some cash; that's a nice incentive until you realize that just buying a small bag to hold additional items costs about 30x the typical chest loot. In Crackdown, every time you grab a green glowing orb your Agility stat goes up a little; you can also raise it by winning races and shooting punks from the high ground. Grand Theft Auto IV has you kill 200 pigeons in exchange for an Annihilator Helicopter spawn point; the same chopper is also available in five other locations without shooting all those pigeons. In MMOs, collections are either assembled from random dead-monster loot drops ("Collect ten rat fangs!") or by finding them in the environment (EverQuest 2's "Find the twenty different butterflies!" quests) and either way they give you XP for completing the quest and possibly a quest item.
I'm going to posit that the incremental advantages given by Assassin's Creed 2 and Crackdown are the way to go. Getting a tiny benefit for something you do frequently is satisfying in a trail-of-breadcrumbs kind of way. The fact that you can take the accumulated benefit and occasionally turn it into a useful item (by spending the money you find in Assassin's Creed 2) or that at major thresholds of Agility you gain a new power (as in Crackdown) means that there are bigger breadcrumbs interspersed with lots of little ones, which neatly marries the "periodic plateaus of power" game design philosophy with the "always be grinding" approach, resulting in a situation where every item you collect gives you a tiny hit of happy progression but every so often the same action gets you a much bigger result. This experience really satisfies.
Unlike, say, GTAIV's pigeon-collecting, which practices a Puritanical delayed-gratification ideology: you get nothing until you kill the 200th pigeon and then you get something that you can already get in five other places for free.
For that matter, I'm not a fan of MMO collection quests either. Awarding XP isn't nearly as satisfying as cash (which is liquid and therefore instantly useful and rife with fun trade-offs) or incremental ability improvements (where you are always getting better instead of waiting until you grind up to that next level).
So here's our happy model thus far: in my hypothetical game, you collect items found in the world and every one you grab improves an ability score. At regular thresholds of those ability scores, you unlock a new power or major improvement to the ability. Every item you grab boosts you and at intervals you get a major boost.