Who is "we"?
I would rephrase it like this to avoid it:
"... with all the associated features that are nowadays taken for granted, such as ..."

I would use "OpenZFS" here, to make it clear that it is not Oracle's ZFS implementation we are spporting.

s/uses single/uses a single/

s/in/into/
s/fine tuning/fine-tuning/

Maybe the word 'complete' is better than 'full-fledged'

<emphasis>Fully integrated ZFS support</emphasis>,

including root-on-ZFS, ZFS Boot Environments,
fault management, administrative delegation,
support for jails, &os; specific documentation,
and system installer support.

maybe just 'kernel APIs', since it covers more than just drivers, and we are mentioning drivers later in the sentence

<emphasis>Simple, consistent repository structure

  and build system</emphasis> - &os; uses a single
repository for all of its components, both kernel and
userspace.  This, along with an unified and easy to
customize build system and a well thought out development
process makes it easy to integrate &os; with build
infrastructure for your own product.

/in extending/into extending/

I'm not sure about this one. "Kernel API" can mean the API that the kernel provides to the userspace; "kernel driver API" makes it very obvious that it's about the API for stuff that runs inside the kernel.