Adaptive Path’s Andrew Crow on peer generated achievements in Halo 3:

In large games, teams usually self-organize based on skillset. Some players are great snipers, others good with a rocket launcher, etc. Once you are known for your skill, the other team members will leave those valuable weapons to you, because, in your hands, the team will benefit. Sometimes people will even bring weapons to you so that you can do your thing. This affordance is a reward for the respect that team members have for one another. It's not measurable, but it's valuable.

I love this observation, and in particular how this kind of thing could be built into our interactive products.