x86_64 and no SPARC.
Believe it or not, Linux is not the end all & be all OS. There are things that other OSes do differently from Linux that might have application for real use cases.
vxWorks is a commercial real time OS uses on the mars rovers. NASA had a reason to choose it.
Contiki is another embedded OS that does things Linux cannot.
OpenBSD does security and code review. Some of it has trickled to other OSes. (W^X)
OSX runs a Mach microkernel with a BSD derived OS.
Minix is still around too.
VMware and Android both use a Linux derived kernel but do not look like Linux.
Even Windows has its place.
I currently work with OpenStack which (mostly) means Linux. I've been using Linux since '92. But I was a Solaris admin until recently and even installed Solaris 11 a few times. There are some things Solaris does better than Linux.
I've found Solaris to be more stable and better at handling loads. I had apps that ran fine on Solaris that crashed Linux on the port.
Dtrace is an awesome tool to see what is really going on with your app. Systemtap might get there.
Zones are secure, reliable containers. It's nice to finally see them get used in Linux. It will be good if they get the security up to the level of Zones.
ZFS, well it's already on FreeBSD and I've been using it for years with ZFS on Linux. I'd like to see btrfs at the same reliability. I wish *every* CLI had as good a UI as ZFS does. I'd love a GUI that was as good!
Solaris switched to SMF from SystemV type startup a long time ago. I liked it better than upstart. Systemd has been a bit smoother than SMF was at first.
Would I use Solaris for a desktop? NO! unless I had no choice. I bet most users stopped using RHEL/CentOS in favor of Ubuntu or something else a long time ago.
Oh, forgot KVM inside Zones for OpenIndiana.
Run Linux on KVM inside a Zone. Use dtrace on your Linux binaries.