Not much is that special in terms of the XEPs (extension protocols) that they have defined. When you innovate with it though, man you really see the power of correct XML.

You can slap custom elements pretty much anywhere you want, as long as you have your own namespace (and it's recommended you only place them under <message> or <iq> elements). Say you have some proprietary technology in a client application, with XMPP you can throw an element under the <message> that your client can recognise and act on. For everyone else provide a hyperlink within the <body> element and serve up a web page for them. If they are using your client "bam!" instant added functionality - but if they are on device X which you do not support they are not left out in the cold.