Looking at diffs on the command-line is cute and all, but for anything substantial I doubt this will ever be as good as using a proper GUI interface. I like "meld". You have to install it, then run

  git config --global alias.meld '!git difftool -t meld --dir-diff'

and after that you can do:

  git meld                   # like "git diff"
  git meld --staged          # like "git diff --staged"
  git meld branchA branchB   # like "git diff branchA branchB"