Analyzing the upgradation of Rails application from 2.3.5 to 3.0.3

I have an existing Rails application created on Rails version 2.3.5. I now want to upgrade it to Rails version 3.0.3. But first I want to analyze what are the points I need to consider, which are the areas that need more focus.
I found an official plugin for Rails upgrade named: rails_upgrade
To install the plugin run the command:

ruby script/plugin install git://github.com/rails/rails_upgrade.git

The plugin adds the following tasks:

• rake rails:upgrade:check - Check your app for required upgrades
• rake rails:upgrade:backup - Backup your likely modified files that might be overwritten by the generator
• rake rails:upgrade:routes  - Generate a new route file
• rake rails:upgrade:gems - Generate a Gemfile from your config.gem directives
• rake rails:upgrade:configuration - Generate code for a new config/application.rb from your environment.rb

You can also refer to link- "http://omgbloglol.com/post/353978923/the-path-to-rails-3-approaching-the-upgrade" which has documented the upgrade in detailed way.