Cropping is the most straightforward. You pinch and zoom to crop, there are some preset constraints under the gear button menu.
If iPhoto detects something resembling a horizon line, it’ll show a line flanked by some extremely period-correct radial brushed metal buttons to either accept or reject the changes. Today, building this would’ve been significantly easier with the nifty VNHorizonObservation class in the Vision framework.
Tapping the straighten angle knob kicks in the gyroscope, so you can tilt your device to get the exact angle you want. We don’t see a ton of these sorts of interactions that make assumptions about hardware capability and the user’s motor skills. Maybe that’s for the best?