You're not really understanding how ZFS does and can work. It already has hooks to provide 'features' such as you talk about. It does require crossing several traditional Unix boundaries, thats true, but its an accepted trade off to get the benefits that go with ZFS, but the hooks to include such features at the typical boundary points still exist in the ZFS code. Pretending that ZFS has to be totally and completely aware of what you hook in isn't really fair. What you hook in has to integrate with the API, which is well defined, and that really isn't any different than with the approach you seem to prefer.
And for reference: dm-cache and cache are not needed with ZFS, l2arc already covers them, and it does it better because it knows whats going on across all 3 layers. I seem to have no problem doing iscsi sharing of ZFS storage space nor do I seem to have any problem using iscsi targets as part of zdevs. Hell, technically you can still use dm-cache and bcache with ZFS, if you're ignorant enough to do so. You can even run whatever file system you want on top of zvols. You'd be stupid to do it in most cases, but the ability is there if need be.
Since you want to use the word Unix, lets get a few things clear. Linux is not and likely never will have a Unix certification. Sun on the other hand had two operating systems that were certified Unix and they were doing it before Linus had a computer to start Linux on. Drop the 'my OS does it right' bullshit because your OS isn't what you're claiming it to be, and the system you arguing against was written by people who did make something you're claiming it isn't.
I don't disagree with the Unix tradition in the least, compartmentalized code with strong boundaries and good interoperablility where ever possible ... and occasionally you tear down the walls for specific reasons. Graphical performance is an example where your philosophy sucks, which is why Windows kicks the ever living shit out of Linux performance. Note: Linux, NOT Unix. SGI had a terrific graphics stack as an example, and Sun's wasn't too horrible.