Well, the general idea is to ensure that whoever is using Opa somehow contributes to the FOSS community, either by contributing source code or by funding the development of Opa itself.

How is that limiting for startups? Most startups are built on execution and network-effects and I don't see how the license is a hindrance to that.

Really? Source-code is not part of the execution?

A vast majority of start-ups, talented developers see afterwards and go "Oh, I could have coded that", but perhaps not built the business or had the drive to go after the users. They can see the site and imagine how they would reverse engineer it. The value is in the user-base and execution of the business. Only a few start-ups rely on what might be a non-trivial new technology that was invented in house, for which closed source is necessary.