What is reputation? How would you explain it? How does it aggregate?
Draw some pictures - what might it look like, what might it be integrated into a site? Etc.
In my mind, reputation is the summary of a series of transactions. Ebay's user reputation comes directly to mind. How they will interact across sites seems challenging -- I have 1000 points on news.yc and 2000 followers on twitter; how do they interact? How do you present this, etc?
I recommend you cut the idea to the very minimal core to see how small you can make it, and see if you can validate that minimal concept.