What Sam suggests is that aggregators such as FeedDemon inform of validation errors only when first subscribing to a feed. After informing the user of the fact that these errors might affect how the feed displays, the aggregator should continue to subscribe and not show any further indication that the feed is invalid. The hope is that this would gently persuade feed creators to pay attention to validation problems without stopping end users from reading invalid feeds.
After accepting that Sam's idea makes more sense than my own (which took all of two minutes!), I started coding this into FeedDemon. When a user subscribes to a feed, FeedDemon checks it for well-formedness, and if any problems are found the dialog shown below is displayed:
Clicking the "Details" button shows the text of the validation error, and a link to the online Feed Validator provides a way for authors to check the feed for additional problems.
My hope is to include this in the next build of FeedDemon. As always, I'm open to other suggestions, so please post a comment here if you think this can be improved (or if you think it's a bad idea altogether).