I just switched from an iPhone 4 to an S4. I got sick of waiting for a worthwhile upgrade from Apple. It will take more than a 5" screen to get me back - it will take the following:
- Real customization features
- Widgets - would be nice but not a deal breaker for me
- SBSettings-like feature out of the box - iOS 7 adds a few options, maybe we will see more in time.
- SD slot - won't happen, but would be nice.
- user-replaceable/upgradable battery - same as SD card.
- USB host mode (I love that I can plug in a keyboard and mouse and actually use it much like a PC) - Bluetooth seems to be the favoured option for Apple.
- Compatibility with standardized devices, such as MHL 2.0 interfaces
- 1080p resolution
- 2 GB RAM
- "virtual" buttons in addition to home (like Android's Back and Menu buttons)
- Better task management
Oh, and the ultra-high ppi display is amazing - I can read full-page PDFs because the characters can be fully drawn, not just be blurred approximations. Reference manuals, e-books, etc. all tiny fonts fully readable whereas on the iPhone I had to zoom to a column of text for it to render. I'm re-reading an RHCE study guide, puny yet fully readable text on this 4.99" 1080p screen.