XPATH is your friend. Try this and forget about that crappy xlink syntax :-)

HtmlNodeCollection tl = document.DocumentNode.SelectNodes("//p[not(@*)]");
foreach (HtmlAgilityPack.HtmlNode node in tl)
{
    Console.WriteLine(node.InnerText.Trim());
}

This expression will select all P nodes that don't have any attributes set. See here for other samples: XPath Syntax