FeedBurner is pushing the envelope in terms of making individual feed items useful with their new product FeedFlare. Since FeedBurner proxys feeds for publishers, they can easily add elements to a feed item such as related links (according to Technorati) or tags from del.icio.us. I wonder how far this can be pushed, because while a feed item is generally rendered as HTML, aggregators aren't quite the same as web browsers.
One downside though is that aggregators need to be able to remove the FeedFlare stuff before doing duplicate detection. Seems like Bloglines isn't doing this and let's just say the results are a bit irritating. More work for aggregator writers if this becomes popular.