[filereaper]

I think Simon nailed it with the points under this heading "Reliance on edgy kernel features"

Officially Docker is only supported on RHEL 7 and up, and most systems I've seen are still on RHEL6.

I think its just a matter of time before Docker goes into Production, where I'm working we're seriously looking at "Dockerizing" lots of things, but OS support keeps popping up.

[e40]

This, a thousand times. I'm on CentOS 6 and have switched to docker for some services that don't easily run on that OS. However, I found out the hard way that docker isn't supported on CentOS 6, by having some crashes when building containers (due to bugs in devicemapper). Painful.

I really wish RH had found the time to fix RHEL 6 and support docker.

RHEL 7/CentOS 7 is a big step for many. RHEL 6 isn't even near EOL and many people (including myself) wanted to get more mileage out of CentOS 6.

[Rapzid]

Most systems I see run debian or a debian based distro(such as Ubuntu). By comparison to RHEL, they are all running with "edgy kernel features".