I think the main problem is going to be getting people to develop for an alternative OS. MS had a paid developer per unique app program, which was called desparate by some. Comments here state that license to develop for the WP is a $100??? Jeez, I wonder what their growth team is thinking.
Developers are highly sought after, experimenting is fun, but a lot of people are looking for stable jobs either from the beginning or they want to try a start up / own their own business first. This is also a common problem with trying new programming languages, why choose a new one when established languages have large user base, jobs are plentiful, and not much worry it's going to be abandoned 2 years down the line?
There has to be incentives to overcome these barriers to gain large spread adoption. At the end of the day a lot of it comes down to livelyhood. Will a person risk time devoting on something new? If a person starts a company on this new OS, will other developers take that risk to join? Will the clients/customers be willing to make that switch?