This is most probably an namespace issue. Try to add:
xmlns="http://purl.org/rss/1.0/"
as namespace to your xslt stylesheet.
For example the following xslt:
<?xml version="1.0" encoding="iso-8859-1"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:rss="http://purl.org/rss/1.0/">
<xsl:template match="/">
<items>
<xsl:copy-of select="//rss:item" />
</items>
</xsl:template>
</xsl:stylesheet>
Will generate the following output:
<?xml version="1.0"?>
<items xmlns:rss="http://purl.org/rss/1.0/">
<item xmlns="http://purl.org/rss/1.0/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:slash="http://purl.org/rss/1.0/modules/slash/" xmlns:dc="http://purl.org/dc/elements/1.1/" rdf:about="http://sometld.tld/dokuwiki/doku.php?id=interessante_und_hilfreiche_links&rev=1368016122&do=diff">
<dc:format>text/html</dc:format>
<dc:date>2013-05-08T14:28:42+02:00</dc:date>
<dc:creator>akku</dc:creator>
<title>interessante_und_hilfreiche_links</title>
<link>http://sometld.tld/dokuwiki/doku.php?id=interessante_und_hilfreiche_links&rev=1368016122&do=diff</link>
<description>
* .NET Framework Setup Verification Tool <- This .NET Framework setup verification tool is designed to automatically perform a set of steps to verify the installation state of one or more versions of the .NET Framework on a computer. It will verify the presence of files, directories, registry keys and values for the .NET Framework. It will also verify that simple applications that use the .NET Framework can be run correctly.
</description>
</item>
<item xmlns="http://purl.org/rss/1.0/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:slash="http://purl.org/rss/1.0/modules/slash/" xmlns:dc="http://purl.org/dc/elements/1.1/">... more items ... </item>
</items>