What are the minimal requirements for successful gamification? I am very interested in the concept of Gamification, the idea (used here on Stack Exchange) that by making mundane tasks into a game, you can elicit desired behavior from users of software. (For example, by giving people arbitrary reputation points, you encourage them to help the best answers rise to the top...)

Wikipedia says that some of the techniques used for gamification include:


  
  achievement "badges"
  achievement levels
  "leader boards"
  a progress bar or other visual meter to indicate how close people are to completing a task a company is trying to encourage, such as completing a social networking profile or earning a frequent shopper loyalty award.[7]
  virtual currency
  systems for awarding, redeeming, trading, gifting, and otherwise exchanging points
  challenges between users
  embedding small casual games within other activities.[2]
  


I'd like to see what studies actually say about gamification. If I wanted to start a new website which used gamificiation to entice users to participate, which techniques are the most efficient? Do some techniques work better than others? Do you need multiple rewards (E.G. reputation and badges)?

What do studies show are the minimum requirements for successful gamification?
In short, I'd say the minimum requirement is to understand which behaviours you are influencing and why.

Before you consider which game techniques to use, consider the 'story' of the product/service. 

Each story includes:


Actor/s. Who are your targets?
These actors are motivated by something. This is what your game should leverage.
There is a goal. What are you/the actor trying to achieve. Help them progress towards the goal. Motivational factors / game techniques should encourage behaviours that move towards this goal and discourage behaviours that move away from it.
The actor begins in a context. Understanding the context gives you clues to opportunities and constraints.
The actor progresses through a journey (or arc). Mapping this journey helps you know when to employ certain techniques.
An outcome is achieved. This is the most critical part of course. 


Draw this out in a path. Include the diversions, obstacles etc. Identify the main path as well as the little loops of activity that contribute. 

Tip: If this isn't clear consider well known kid's stories. They are all shaped by this familiar pattern. 

The techniques you list above and ones you will see elsewhere are valid. However by considering the story you will understand when to use certain techniques and how they should be approached.