I try to stick to “default is better than configuration”. But there’s some stuff I absolutely must have on a Mac to be useful.
- Launcher (Alfred)
 - App Switcher (Witch) set to nonmiminized windows only.
 - A way to send tasks to rememberthemilk.com from Alfred
 - bunnylol plugin for Alfred (internal to Facebook)
 - Set Desktop to “Sort By Kind”.
 - solarized theme for iTerm2
 - Make left Alt work as +Esc in iTerm2
 - atom with vim-mode and ex-mode.
 - MacPorts installing gpg for passwords, apache2 and MySQL for webdev.
 

$ defaults write com.apple.finder CreateDesktop false && killall Finder # instead of Sort by Kind just hide all Desktop icons
Use Ctrl-f7 or the Keyboard prefPane to give Full Keyboard Access so that tab takes you to every button in a dialog, etc.