Stability
-No registry clutter
-No startup clutter
-No toolbar clutter
Little to no virus risk
iMail - a great Outlook alternative, integrates well with Gmail
iCalendar - same as above plus supports syncing mulitple Gmail calendars
iCloud - great way to backup and sync your Apple devices (requires Lion OSX)
iLife - comes with New Macs / only $30
iMovie - Simple but full featured video editing
Garage Band - Simple but full featured music production and editing
iPhoto - Simple but full featured photo editing
Magnetic power connector - Prevents power jack damage
AppleCare - Excellent and affordable warranty and support directly from Apple