Rome 1.0 does not follow the W3C Feed Validator recommendations.

"A channel should not include both category and dc:subject"
"A channel should not include both pubDate and dc:date"
"A channel should not include both language and dc:language"
"A channel should not include both copyright and dc:rights"

input:
================
public class TestPodDate {
public static void main(String[] args) throws Throwable {
SyndFeed feed = new SyndFeedImpl();
feed.setFeedType("rss_2.0");
feed.setTitle("Title");
feed.setLink("http://www.link.com");
feed.setDescription("Description");

feed.setPublishedDate(new Date());
feed.setLanguage("en");
feed.setCopyright("Copyright");
SyndCategory c=new SyndCategoryImpl();
c.setName("Category");
feed.setCategories(Arrays.asList(new SyndCategory[]{c}));

new SyndFeedOutput().output(feed,new OutputStreamWriter(System.out));
}
}

output:
================
<?xml version="1.0" encoding="UTF-8"?>
<rss xmlns:dc="http://purl.org/dc/elements/1.1/" version="2.0">
<channel>
<title>Title</title>
<link>http://www.link.com</link>
<description>Description</description>
<language>en</language>
<copyright>Copyright</copyright>
<pubDate>Tue, 27 Apr 2010 15:13:45 GMT</pubDate>
<category>Category</category>
<dc:subject>Category</dc:subject>
<dc:date>2010-04-27T15:13:45Z</dc:date>
<dc:language>en</dc:language>
<dc:rights>Copyright</dc:rights>
</channel>
</rss>