I think that the XML syntax of XSLT itself was only one barrier for people to adopt XSLT, even in the XML heydays.

The main obstacles appear to me that the execution model is hard for people to graps; you need to think both as a parser (apply-templates/) and in a more declarative style at the same time. The XSLT can be in a completely different order to the document and in fact it can visit nodes in the document multiple times, hopping through nodes in a different order each time, with lovely constructs like apply-template with "mode" and "select" mixed with call-template by "name", plus you get to use xpath and for-each to boot. The control flow changes from the order in the input, to some predefined order, and back depending on when you decide to match(-template) or for-each. There's a lot going on at the same time! Fun times!