1) A per-article comment RSS feed would be useful.
2) All RSS feeds should be advertised in link rel="alternate" in your HTML head section, not on some other page.
3) Information about RSS feeds and email notification should be included near the comment form that I'm currently typing into.
Charging extra for email notification is your prerogative, of course, but as it is not a paid feature on any other blog/social networking site/etc that I know of, the fact that you are charging for it kind of makes you stand out and I find it a disincentive to subscribe. I find myself thinking, "What? I have to PAY for such a basic thing? Forget it!"