1. Who arbitrates in cases where people bet on poorly worded propositions, or where the outcome either can't be confirmed or falls outside the decision space? Is there going to be a public vote on the result, or are disputed bets made void -- and if so, who decides which disputes qualify to void a bet? Stupid example: "It will rain in Paris tomorrow." 25 people bet each way. It doesn't officially rain, but of the people who said it would, 5 of them agree it drizzled over some neighborhood for a few minutes. The bet didn't specify it had to be according to the meteorological society's website. Will bets require an officially published source? Even if everyone votes down the person who started the bet, it doesn't solve the 50-person dispute now going on.
The creator of the proposition is in charge of arbitrating it.
In your example, the person who created the event "It will rain in Paris tomorrow" will be the only one who will be allowed to decide whether it rained or not.
So there is a reputation element in the event creator.
2. How are you going to make money off this?
Me? I won't. If the service is usefull, I may make money by using it.
3. What you're proposing, if I get it, is a kind of escrow service that uses the built-in features of Bitcoin to provide both anonymity and (by keeping keys secret) disbursement at the right moment. Why is a totally unmoderated prediction market necessarily a better thing to start this service with than, say, a sportsbook, a foreign exchange or shadow stock market, where the results can be computed automatically from publicly available sources? Choosing a particular market limits the number of things that can go wrong, and would probably help coalesce the critical mass you're looking for.
I don't want to moderate anything as I think it is too much work. I want a 100% automated website.