I think the concept of Opa is neat. Other projects may have tried and failed at this, but maybe the Opa authors could make it work.
However, the choice of license completely precludes me from even trying it. Sure, I release source code for some of the stuff I make (even though nobody looks at it). Here's why:
Let's say I try out Opa, make some side projects with it, fall in love with it, and I get good at it.
Now, either at my day job, or on my own, I come up with super awesome project X that I want to build and release as some sort of money-making venture. We may even want to open-source the code for the site eventually, but we're not sure yet.
If I were to leverage Opa to do this, however, I will have to *pay* to keep my source closed. That's just not acceptable.