YAML is more complex that most people tend to realize. (This was brought up in a 2011 discussion about possibly standardizing a metadata section for Markdown documents which sadly went nowhere. [1])
Take a look at example 2.11 in the YAML spec [2], for example, and see if you can make heads or tails of it.
You don't need most of those features. A pared down YAML with the cruft removed (implicit typing, flow style, tag tokens, node anchor & references) is actually pretty simple as well as less "gotcha-y".
Okay, does anyone actually believe that causes a problem?
I can think of ONE time when that causes a problem, and that's with indentation with multi-line strings. Oh look, HJSON included that feature. That's like throwing the baby out and keeping all the bathwater.
I can't find a specific example off the top of my head but I'll say I've been managing a Jekyll site for a while now and whitespace errors in frontmatter and data files cause all kinds of problems. I'm not sure I could explain the details but it's a legit criticism of YAML. IMO part of the problem is that YAML looks very straightfowrard and is until it suddenly isn't. Whitespace is part of that problem.
As an anecdote on the flip side, I've been building Middleman sites for a while now and can't remember ever having an issue with whitespace in the front matter or local data.
Unquoted strings are valid in yaml just like this format. There are at least 2 ways to specify a list of things. There are some super bizarre looking possible formats for lists of mapping types.
There are a number of others given the length of the spec. Yaml is a complicated beast that generally has more than one way to do any given thing
http://www.yaml.org/start.html