Yesterday, I wiped my GNU/Hurd partition clean and tried reinstalling the system (gnumach-1.2-1, hurd-19990725) with the cross-install script. Everything looked fine until the Hurd booted:
ext2fs: hd0s3: warning: mounted readonly because of unsupported optional features (0x1) ext2fs: hd0s3: panic: main: no root node! Apparently mke2fs 1.15 has been changed to set the "sparse superblock flag" by default, and the ext2fs server of the Hurd doesn't cope with such file systems. I tried giving the -s0 option to mke2fs, but that didn't change anything. I added -r0, and now mke2fs listed a few more backup superblocks. Believing I had fixed the problem, I installed the packages and booted the Hurd. The warning message was gone but the panic was still there. At that point, I gave up and downgraded mke2fs to 0.14, which just works.
