Ubuntu backports the kernels for the next four releases after each LTS. If you're on trusty you can simply install kernel 4.2 by installing the linux-generic-lts-wily package.
Once 16.04 has been released the 16.04 kernel will be backported to 14.04, then the cycle repeats and we start getting backported kernels to 16.04 every six months.
Lastly, as others have pointed out, where does one get a "server" with less then 16GB ram?
I have an old Sun Ultra 2 server still running. I think it's close to 20 years old now. It has 512 MB of RAM, runs ZFS on Solaris 10. It's doing just fine.
Doesn't the operating system already have one?
Yeah it has one, the one in the C library.
CentOS has to wait until Red Hat releases their source RPMs, then they have to rebuild them, test them and distribute to all their mirrors.
Pay for a RHEL subscriptions if you want your patches fast.
RHEL/CentOS 7 used to have GNOME 3.8 and I agree that was far from perfect. Red Hat recently updated it to GNOME 3.14 in the RHEL/CentOS 7.2 update, so that should have fixed most of the issues. Sure if GNOME just isn't for you then that's a matter of taste, it's not something wrong with GNOME by itself.
By the way, try sudo yum groupinstall 'KDE Desktop' if that's what you want.