One of my guiding principles with Handlebars was explicitly to avoid adding any new features in the template itself that were not ways to reference a part of the JS. Normal Mustache gives you one simple way to reference a key in the current context--handlebars adds an extended path syntax and a mechanism for passing a context to lambdas (which already exist in Mustache).
Interesting. I'd actually like to see Mustache go in the direction of being a bit more like django templates by adding filters, which I think are no more likely to confuse a designer than plain old mustache tags.