The GNU site describes the history of HURD's experimentation w/ different microkernels. It doesn't explain why L4 failed - it just somewhat tersely states that its work stopped in 2005, after which 2 more microkernels - Coyotos and Viengoos - were tried out.
As I've noted elsewhere in this page, they could have simply forked Minix 3, which is small and well documented, and gotten a microkernel that provides all the services that HURD requires. After all, Minix is modelled on Unix and provides the microkernel underpinnings, while HURD provides the upper layers that consume the microkernel services. These 2 could have been a beautiful combination if not for the clashes b/w the licenses.
Only problem is that HURD doesn't support SMP. Nor x64. Otherwise, an i7 based system would be ideal - it has all the rings of security that HURD needs, the multi-cores would help w/ the message passing & the context switching, while the x64 would make it relevant by not limiting it to a 4GB address space.
As far as the file systems go, I can't think of ONE that is GPL3, which is what HURD would presumably need. I mean, HURD is GPL3, GNU userland is GPL3 but suddenly, the file system is just GPL2? Even the latest file systems for Linux - be it Btrfs or ext4 are not GPL3. Hopefully, they come out w/ one that doesn't suck