It is implemented. Open an article you want to subscribe to, and use "Subscribe to RSS Feed" on the right.
Looking at the features page, only about half of the features are applicable to Less Wrong. Of those that are, most of them seem like things that we should add to Less Wrong itself rather than having them only be usable if you download an addon.
What should be added to Less Wrong, and what will be added to Less Wrong, are different things.
So what's the status on this? Are the votes sufficient or insufficient to try an implementation of RES for LessWrong?
If anyone cares, I could probably port this to work on LW without too much trouble. Optimistically it'd just involve opening up the source and replacing reddit.com with lesswrong.com. More realistically, there'd probably be a lot of baked-in assumptions about DOM structure that'd need to be updated to have the UI enhancements make sense.
Anyway, this is mostly just a straw poll to see how many others would be interested in such a thing.