Because the world could not have possibly gone on spinning without another responsive CSS framework.
- Support right-to-left languages without any markup change
- Make nesting painless
- Provide useful but minimal configuration options
- Ditch .first/.alpha and .last/.omega in favor of :first-child and :last-child
- Better documentation
- Push/pull/offsets
- Variable number of columns
- Specific features for mobile screens
- IE7 support (maybe)