I subscribe to a few feeds I'll leave unnamed that are the product of very smart people and/or organizations, but their software gets one annoying thing wrong. Every time there's a new comment on the post, the original feed changes and my reader re-highlights previously read entries to indicate a change.
Sometimes the change is limited to a count of the number of comments on the post.
Sometimes the change is to tell me so and so commented on it.
Sometimes a timestamp changes, that's it.
My feedreader isn't so smart that it can tell substantive changes from inconsequential ones. Readers of the future will be able to do this, or at least let users configure them to ignore certain tags. After all RSS (and Atom) is just XML. Meanwhile my reader is smart enough to find when one blog I subscribe to is commenting on another item in my list.
We're still in early days in this technology, still in the first decade, depending on whom you listen to. We'll get it right eventually.