I love the XBox Live Gamerscore system. It's great. A persistent and public souvenir system was a wonderful idea and one of the reasons that my XBox 360 gets so much love.
It just frustrates me how little thought goes into the system by some developers.
Gamerpoints are not separate from a game. They are a surprisingly well integrated part of the 360 experience; you can't consider a 360 game separately from its points system. Each and every 360 game has this list of goals, with a point value next to each goal.
These are not sub-goals. This is the first place developers go wrong. These goals are not subordinate to the goals set by the gameplay. The player experience is not about reaching the end credits first and earning gamerpoints second. This is because of how achievements are worked into the 360 interface. A player can view the list of achievements for a game at any time after they have first launched it; if they have a friend who's played it, they can see the achievements by accessing the friend's gamercard, whether they've played the game themselves or not. The achievements, and the overall number of points earned for the game, display every time that a player reviews their own gamercard.
Achievement points are the over-goal in a 360 game. Finishing the core game experience is a stepping stone in the task of getting a perfect point score. Achievement goals are reinforced before, during, and after gameplay, and persist after a player loses interest in the game.
What that means is that each core gameplay goal should be reinforced with an achievement. There should be an achievement for completing each level, one for finishing the game, and one at the end of any collection task. For large collection tasks, there should be achievement points at each meaningful milestone along the way.
Achievement points should not reward the player for bucking the core game experience. Don't give out points to players for playing in a non-standard way. You may be rewarding a small percentage of players who like doing this stuff, but you're also encouraging a much larger percentage of players to spend time experiencing your game in a sub optimal way.
Examples: Guitar Hero III gives out points for playing through campaign with a standard controller instead of the guitar. Dumb. Penny Arcade gives out points for finishing the game without having any of your characters knocked out during combat (necessitating a huge number of unnecessary reloads). Dumb.
I'm not saying don't encourage the player to experiment with new play styles. That's great. Just don't ask them to sustain those play styles if they're not the strength of your game. In the above examples, GH3 should have given the points for finishing a single song with a regular controller, and Penny Arcade should have awarded the points for completing, say, three consecutive battles without a death.
Weight core gameplay goals higher than ancillary ones. The lion's share of rewards should be doled out for playing the game in the way that was intended. Esoteric or very difficult challenges should have only token rewards. Let's look again at Guitar Hero 3, which gives out only a token number of points for finishing the Easy and Medium difficulty campaigns, while shelling out the big bucks to the 0.05% of players able to finish Through The Fire And Flames on Expert. Let's face it - a player who spends the time to overcome GH3's nigh-on-impossible upper echelons is no longer motivated by achievement points. From my experience with the game, the biggest milestone for most players is finishing Medium, and that's something that should be appropriately rewarded.
Achievement points can be a fantastic tool for directing and focusing player effort on the best experiences that your game has to offer; using this system to funnel players into your lame sidequests and hokey minigames is just plain stupid. Think it through, people.