I like this! Very smooth and minimalist interface, and I love the idea of posting your reasoning when making a prediction. I found it interesting reading through the posted reasons on both sides of a given prediction - good way to get an overview of the (presumably) better arguments for both.
My one suggestion would be to add a probability or confidence level slider to the "place bet" screen. While we can presume that someone betting on a given outcome does believe there is "at least a __% chance the answer is ____", it would be interesting to see the distribution of the probability that predictors assign to their predicted outcome.