How can we improve Phusion Passenger? We're very open to feedback from HN readers. If you have any suggestions, please let us know. If you have any questions about it please feel free to ask.
2. Yes. Phusion Passenger 4 fully supports Ruby 2.0 so it doesn't prevent you from using it, but you may run into Ruby 2.0 bugs, e.g. https://bugs.ruby-lang.org/issues/8100. We recommend staying with 1.9.3 until the next Ruby 2.0 patchlevel, but the choice is yours.