as a developer in training:
performance scalability (with technical reasons supporting it) from PhilG's book etc
economic scalability (low entry costs, price scales with project size (because of performance scalability and open source)
customability (freedom to modify as needed, and responsibile for results that go with it)
available resources (open-source communities, documentation)
community collective behavior, on whole, consistent with long-term project vision, priorities, values
open-source (opportunity to contribute to something "big", opportunity to learn from competent quick-fixers and like minded long-term solution providers etc.).