Apart from the sort of features that I now take for granted in a filesystem (data integrity, easy management, extreme scalability, unlimited snapshots), ZFS also has built in compression.
I've already noted how this can be used to compress backup catalogs. One important thing here is that it's completely transparent, which isn't true of any scheme that goes around compressing the files themselves.
Recently, I've (finally) started to enable compression more widely, as a matter of course. Certainly on new systems there's no excuse, at the default level of compression at any rate.
There was a caveat there: at the default compression level. The point here being that the default level of compression can get you decent gains and is essentially free: you gain space and reduce I/O for a negligible CPU cost. The more aggressive compression schemes can compress your data more, but having tried them it's clear that there's a significant performance hit: in some cases when I tried it the machine can freeze completely for a few seconds, which is clearly noticeable to users. Newer more powerful machines shouldn't have that problem, and there have been improvements in Solaris as well that keep the rest of the system more responsive. I still feel, though, that enabling more aggressive compression than the default is something that should only be done selectively when you've actually compared the costs and benefits.
So, I'm enabling compression on every filesystem containing regular data from now on.
The exception, still, is large image filesystems. Images in TIFF and JPEG format are already compressed so the benefit is pretty negligible. And the old thumpers we still use extensively have relatively little CPU power (both compared to more modern systems, and for the amount of data and I/O these systems do). Compression here is enabled more selectively.
Given the continuing growth in cpu power - even our entry-level systems are 24-way now - I'm expecting it won't be long before we get to the point where enabling more aggressive compression all the time is going to be a no-brainer.