Mesa 7.5.1 is a bug-fix release fixing issues found since the 7.5 release.
The main new feature of Mesa 7.5.x is the Gallium3D infrastructure.
Mesa 7.5.1 implements the OpenGL 2.1 API, but the version reported by glGetString(GL_VERSION) depends on the particular driver being used. Some drivers don't support all the features required in OpenGL 2.1.
See the Compiling/Installing page for prerequisites for DRI hardware acceleration.
tbd