Why I am not enthused about Red Hat Enterprise 6
I have to admit straight off the bat that this is mostly an uninformed prejudice. I have not actually run RHEL 6 machines (our few RHEL machines are RHEL 5); all that I've ever done with it is install it a couple of times to test some things. So part of this is based on general knowledge of what it has and part of this is based on those install experiences.
My impression of the install experience was not positive. Although I don't remember details, it struck me as generally less functional and more annoying than the RHEL 5 equivalent. I could make it work but I didn't like it. And of course the capstone of the install experience is that it uses NetworkManager (in a situation that NM is not good for) and then leaves your networks down when you boot the installed system. This means that the very first post-install thing we'd have to do with RHEL 6 is to reconfigure all the networking, ripping NM out and putting the old ways back in.
Beyond that, RHEL 6 is simply built on an awkwardly transitional base because it was done at a bad time. It's based on Fedora 12 plus chunks of 13 and 14, which puts it just before Fedora made a number of important changes such as moving from upstart to systemd, a major change in hotplug device handling, and so on. You get some changes from the old ways of RHEL 5 but they are by and large the wrong changes, ones that would later be abandoned. And you get other changes that were only half-baked at the time of Fedora 12, such as NetworkManager (especially on servers). All of this leaves me unenthused.
We are not big RHEL users here, so my general plan (to the extent that I had any) was to skip RHEL 6 entirely and wait for RHEL 7 for, say, new iSCSI backends. Various things have gone wrong with that, but that's another entry.