The most important feature of OpenGL 4.3 is the introduction of Compute Shaders. This is a simpler way for programmers to process general data using the graphics processor cores. While the compute shaders are limited to the instruction available within an OpenGL shader (in the GLSL language), it is complex enough to perform complex physics computations and leave the results available for OpenGL to use in a graphic operation right away.
OpenCL is another way to perform such operations, and although Compute Shader are more limited than OpenGL, they integrate much better within a graphics application and can be called upon at a much higher frequency.
There are many more API changes, but Compute Shaders are most likely the most important update for OpenGL 4.3. By using the graphics processor (GPU) cores to work on inherently parallel tasks, developers have a shot at adding great effects without killing the application’s performance.