As someone who for years now has used RSS readers to consume news from a variety of sources I can confidently say that the one thing that has bugged me the most is the continued tendency of publishers to switch to partial content feeds. Partial content feeds limit the number of characters available in the feed and typically (but not always) contain a read more link that takes the reader to the web page that contains the full post.
The reasoning behind partial content feeds is very simple. For the most part publishers of partial feeds want to force their audience to visit the website so that ads can be viewed alongside the content. In some cases there are partial feeds due to paywalls and the subsequent authentication that is not supported in a feed. I’m sure there are also a few cases where the publisher doesn’t realize that they are pushing out a partial content feed.