Main reasons for me:

Client-server model

Insufficient online documentation for what seems to be a verbose and somewhat limited configuration language.

The XML is a minor turnoff but for me it's more a question of what the language is doing. Eg:

        <Package name='openssh-client'/>
        <Package name='openssh-server'/>
        <Package name='emacs'/>
        <Package name='curl'/>
        <Package name='rsync'/>

In puppet, you define a list of packages and then install them with one declaration or do any number of other things with that same list.