This is an idea that requires way too much engineering, but I wish it could happen: How cool would it be if Mac OS X had a concept of “subscribed RSS feeds”, so that no matter which browser or feedreader you used, viewing a site’s webpage would not offer you the “subscribe” link if you already were indeed subscribed to it.
At present this would require browsers parsing the formats of whichever feedreader the user was using, which is clearly a rather unscalable approach. If all feedreaders used a common format file (ahem, not OPML) then this would be a lot easier :)