Every improved feature is a a reason to upgrade for someone. There are plenty which come to mind but it depends on where you are starting from and your photography:
- If my camera did not have a 100% coverage viewfinder, it would be the first reason to upgrade.
- A weather-sealed body and matching lenses is essential for some environments. I don't control the weather when I'm on vacation (or ever) but I want to keep shooting.
- More external controls make a body more efficient and help achieve results faster.
- Larger pixels for higher dynamic-range and lower image-noise.
- Higher resolution for increased maximum print sizes.
- Higher maximumminimum shutter-speed to freeze faster action.
- Longer minimummaximum shutter-speed for extreme low-light photography. Even with a BULB mode, many cameras have a maximum exposure-time.
Some features can be built-in for convenience even though they can be added with an accessory too. For example, Interval-Timer, GPS, WiFi. Other features such as HDR and Time-Lapse can be done via software or in camera. Again, what matters is what is important to you and, in the end, you have to compromise too. You can want a full-frame DSLR which shoots at 12 FPS but may not be willing to carry its weight.