I've only played with RancherOS for a very short amount of time so keep that in mind :) I don't think we quite know exactly what it will turn out to be or exactly where it would be most effective. Without going into the technical details (because I'm not the person for that - I'll leave that to rancher), RancherOS isn't just a Docker image. It runs Docker at the core. Not just an OS with Docker in it. This gives it the potential to have an update mechanism similar to CoreOS etc.
You are correct in that it is very similar to boot2docker. boot2docker is also a very light distro with docker (and by "very light" I mean awesome slim -- steeve is amazing :) The main difference, as I see it, is in updating / extending / packages. With TCL, you need to either find the package and include it in your build or create the package with a build chain etc. which isn't trivial. In the RancherOS route, you could simply pull a new docker image.
The interesting thing to me is it offers choice. If you want to run systemd, etcd you can without changing an entire system (i.e. today, if I want Fedora with systemd I have to configure the Docker daemon opts differently than say with Ubuntu).
You are correct in that it is very similar to boot2docker. boot2docker is also a very light distro with docker (and by "very light" I mean awesome slim -- steeve is amazing :) The main difference, as I see it, is in updating / extending / packages. With TCL, you need to either find the package and include it in your build or create the package with a build chain etc. which isn't trivial. In the RancherOS route, you could simply pull a new docker image.
The interesting thing to me is it offers choice. If you want to run systemd, etcd you can without changing an entire system (i.e. today, if I want Fedora with systemd I have to configure the Docker daemon opts differently than say with Ubuntu).