Out of the things Assassin's Creed 2 does a lot better than the first, I think one prominent spot is in it's collectibles.
Now I'm not someone who desires to collect every achievement in every game, as most of the games I've played range from 250-750 in number of points I have collected, so achievements have never really provided me the drive to collect thousands of out of the way items. This is where Assassin's Creed 2 really shines.
The Codex Pages give you health bar upgrades for every 4th page you decode. The Glyphs each have interesting puzzles for you to solve and are located on or near famous landmarks which your quests often take you on or near. Between the two of them, each rewards you with additional story bits, The Glyphs combine to create a hidden scene that has story implications, and the same goes for when all of the codex pages are gathered together to create a secret picture of some sort for more story implications.
So all I'm saying, is it doesn't take many collectibles to make the system good, in fact it doesn't take many, size doesn't matter and bigger isn't necessarily better in this case. A decent sized amount of collectibles for each world/level which are generally easy to find and have some reward beyond just achievement points. now of course this can be changed, say in a game like Fallout or Dragon Age, you could have a smaller amount of harder to find or collect objects which could unlock a power piece of weaponry or armor.
What seems to be the optimal method of collectible items or such in games is that the amount of collectibles is equal to or less than the value of the rewards it provides, as well as being placed in interesting areas that you are likely to visit either on account of the gameplay or the world/level's beauty.