No route matches [GET] /assets

In production mode, Rails will not be responsible for serving static assets. Therefore, you are getting this error. Thin won't do it either, since it's just a wrapper around Rails.

This is controlled by this setting in config/environments/production.rb in your application:

config.serve_static_files = false

Or in Rails 5:

# config/environments/production.rb
config.public_file_server.enabled = true

Or set ENV['RAILS_SERVE_STATIC_FILES'] to true.

You can either set to that true or use a real server like Apache or Nginx which will serve the static assets. I suspect Pow may also do it.

---

If you're on Heroku, they recommend the use of the rails_12factor gem which enables this setting by default. Place the gem into a production group in your Gemfile, like this:

group :production do
  gem 'rails_12factor'
end

Adding to what Ryan said above, the Rails asset pipeline guide describes how to setup Apache or nginx to serve the static assets for you.

http://guides.rubyonrails.org/asset_pipeline.html

You really should setup nginx or Apache to serve static assets, as they're much better optimized for this task than mongrel/thin/unicorn.