Step 1: Download the "Command line tools for Xcode" here
https://developer.apple.com/downloads/index.action
Step 2: Install homebrew with this command
/usr/bin/ruby -e "$(/usr/bin/curl -fksSL https://raw.github.com/mxcl/homebrew/master/Library/Contribu...
Step 3: gem install rails
brew install rbenv
brew install ruby-build
rbenv install 1.9.3-p125
rbenv global 1.9.3-p125
gem update --system
gem install rails
gem install bundler # optional, recommended
brew update
brew upgrade
Lowering the Rails entry bar is a good thing, making things simpler is a sign of maturity for a product. To address your bit about concurrency and websockets in Rails, people are working on the issue, see the talks given by Ilya Grigorik for example, concurrency will be coming to Rails at some point. And so will the integration with client-side framworks like Backbone or Ember. It just takes time.
A year or two back if you ask, "how do I built my bleeding edge web application" the answer normally was 'use Rails'. My point is the Rails has been sorely lagging behind other invitations.
This just feels like a waste otherwise.