Known Problems of FreeBSD's ZFS implementation

$/!\$ Note that a much revised and newer port of ZFS exists in the 8.X series (8.1-RELEASE, 8.2-RELEASE) and 9-CURRENT, which has so far proven to be significantly better. Some parts of it were backported to 7-STABLE. Because of this, the list of 7.X issues that was hosted on this page earlier is obsolete and was removed.

New items will appear here as problems in the new code are encountered.

Note that i386 and other 32-bit architectures are still definitely not recommended for use with ZFS due to address space issues. If you still want to go that way, please read the ZFSTuningGuide.
tmpfs(5) isn't recommended for use with ZFS because it can suddenly run out of space due to interaction with the ZFS ARC. http://lists.freebsd.org/pipermail/freebsd-stable/2011-January/060867.html