I pay you $1000 (or EUR) if you show me how you can be 3x faster from idea to live with RoR against something else, say Java/Play. If 50% of your time is HTML/CSS fixing,thinking,finding edge cases,testing,debugging,communication with customers etc. you need to be 6x faster. And not in the first 5 days but you need to maintain this speed even as your code size grows and more and more time is spent on understanding existing code.

The only way to reach 6x is with 3rd party libs you have in RoR but you have no equivalent on say the JVM. Massive code reuse is the only major productivity gain today.

[edit] Sorry my mistake. If 50% of your time is taken by other tasks when writing say Java code, the most you can achieve is doubling your productivity by reducing development time to zero. If you tripple your productivity during development, your overall gain is 33% I think (not that good with on the fly calculation obviously).

[edit2] Just realized, this is Amdahl's law for developer productivity with your other tasks being the sequential fraction.