An important one is that XML does not map cleanly to any programming language's typical data structures, even those that explicitly support it. It always turns into an elaborate object hierarchy with awkward query syntax. The combination of unique named string-only attributes with ordered, non-unique named child objects is awkward and ambiguous (i.e. should that property be an attribute, or a child?). Contrast for example with JSON, which translates directly into arrays and hashes, structures available in any decent language.

E.g. you have <user name="john" address="123 Oracle Road"></user>

Now you decide you need multiple addresses per user. Do you: a) invent an ad-hoc string serialization scheme for the address="" property, or b) completely restructure your XML into:

<user name="john"> <address>123 Oracle Road</address> <address>456 XML Lane</address> </user>

But in that case, why not just make every property a child?

XML also spawned idiotic things like XSLT, which most agree is pretty terrible. If you try to use XSLT for HTML templating, by definition you have to escape every meaningful HTML character like "<>&'". If you wish to print one of those characters in your templates, you have to double escape them. No designer I know of would touch it with a ten foot pole, and every programmer I've met who loved XML and its associated technologies was terrible at their job.

Basically XML is strongly associated with the bloaty enterprise Java world, who used XML as their hammer to beat down anything remotely nail-like.