Free software/open source software first and foremost is all about developers. The freedom afforded to developers following the FSF definition of software freedom is the reason for the proliferation of choice at every layer of the stack:
kernel, graphical toolkit, command line shell, terminal program, text editor, office suite, window manager, desktop environment, sound system, etc. etc.
Linux distros is evolving from the functionality stage to the next level which is usability. Devx.com has an advice:
To compete head-to-head with Microsoft, Linux advocates should standardize the platform.
For a subjective comparison of the Big 3 Linux distros, download a LinuxCon 2010 slide.