One of my favorite features in FeedDemon is the way you can change the style used when previewing news items. Given my history with TopStyle you might think that this feature relies on CSS, but in fact it uses XSL to transform the RSS feed. While CSS provides control over how content is styled, XSL also enables controlling the content itself - which means that FeedDemon's preview can be extensively customized.
FeedDemon's styles are stored in its \Data\Styles
subfolder, so if you know XSL you can create your own preview styles. Even if you don't know XSL, you can experiment by copying and editing one of FeedDemon's existing preview styles.
I'm fairly new to XSL, so I'm curious what a real XSL-guru could come up with.