I'm sure I've left a bunch of stuff off this list, but this is what I got off the top of my head.
- A very large (and growing) database of built-in algorithms for image processing and computer vision applications
- MATLAB allows you to test algorithms immediately without recompilation. You can type something at the command line or execute a section in the editor and immediately see the results, greatly facilitating algorithm development.
- The MATLAB Desktop environment, which allows you to work interactively with your data, helps you to keep track of files and variables, and simplifies common programming/debugging tasks
- The ability to read in a wide variety of both common and domain-specific image formats.
- The ability to call external libraries, such as OpenCV
- Clearly written documentation with many examples, as well as online resources such as web seminars ("webinars").
- Bi-annual updates with new algorithms, features, and performance enhancements
- If you are already using MATLAB for other purposes, such as simulation, optimation, statistics, or data analysis, then there is a very quick learning curve for using it in image processing.
- The ability to process both still images and video.
- Technical support from a well-staffed, professional organization (assuming your maintenance is up-to-date)
- A large user community with lots of free code and knowledge sharing
- The ability to auto-generate C code, using MATLAB Coder, for a large (and growing) subset of image processing and mathematical functions, which you could then use in other environments, such as embedded systems or as a component in other software.
