Reading through the lengthy debate on the lkml titled "silent semantic changes with reiser4" [story] is a time investment. Comprised of well over 500 emails and growing, I include here a tiny snippet containing a discussion primarily between Hans Reiser, Andrew Morton [interview], and Linus Torvalds. Questions raised include whether or not the filesystem should be ultimately merged into the mainline kernel, and if so how to go about this. Much of the debate is regarding extensions that are currently only available through reiser4, and perhaps not fully compatible with existing utilities. The thread within begins with some coments by Andrew, who suggests that if the provided feature set is the desired direction for the Linux kernel, his preference would be to "accept the reiser4-only extensions with a view to turning them into kernel-wide extensions at some time in the future, so all filesystems will offer the extensions (as much as possible)".
As quoted earlier [story], Hans stressed that it was important that the reiser4 functionality be merged so that Linux is capable of competing with WinFS and Spotlight. The argument was continued by others, and to these followup comments Linus retorted:
"Hell will freeze over before Microsoft does a filesystem right. Besides, WinFS is likely almost in user mode anyway, ie mostly a library, rather like the gnome people are already doing with gnome storage. So there's really no point in trying to push your agenda by trying to scare people with MS activities. Linux kernel developers do what's right because it is _right_, not because somebody else does it."