If you have been around long enough you still remember the world that was excited about XML and templating it using XSLT. As a hindsight it was a horrible world.
Even though YAML is not optimal, it is a human friendly compromise between too verbose XML and machine only JSON. It lacks native templating, leading to funny constructs e.g. with Ansible files. However human kind has made progress and will make progress further, so it is just a matter of time until someone comes up with sane "native templated YAML" and all projects will adopt it.
I also rather liked thorough extensibility. Namespaces were the right idea, despite clunky syntax. Today you can see Clojure doing something similar in Spec.
And while we're on the subject of XML, XSLT and Clojure; I feel like this is the best solution for readable serialization of tree-like data, and an associated ecosystem of tools (to validate, transform etc). Note some nice features for humans, like the ability to comment out a specific node, in addition to the usual line-oriented comments.