As I see it, there are roughly four main related features that Docker brings, though it is far from the only solution to bring these features:

1. The ability to create services in a clean slate environment. Unix processes tend to inherit by default most environment attributes--working directory, open file descriptors, etc.--and having a way to create a process (or group of processes) without inheriting all of that stuff is easy.

2. Virtualization/isolation in a lighter weight solution than a full virtual machine.

3. Checkpointing that allows you to consistently restart an image with a given configuration, even if you go on to horribly destroy it in tests or whatnot.

4. A language for designing how to construct these images (the Dockerfile).

I suspect the popularity of Docker mostly comes from the latter, and I'd definitely like to see something like that but utilizing jails and ZFS features instead.