Yeah I couldn't really tell what the point was until adding a page. Then since it wasn't really a page that updated I didn't get it. Have an about page that shows a feed for a commonly known page. Something that changes often. That would make it really clear what your product does and if I would find it useful. Since this is YC I went a head and added a page and played around to see how it works, but in the real web I wouldn't take that much effort to try to understand the point of the product.
Once you see it working it is a really cool. It would be useful for some of the sites and blogs I read that still have no RSS feed. Nice work, I like that you keep it simple, but if I want to know more and I am willing to click to learn more you should offer some other information.
The watch changes button made me expect that the page would show me real time changes. Why not change it to "Create Feed" as thats what it does.
You have a lot of white space to work with. Use it to better explain what the site is about. Display the pro account option more prominently. Add a little color.
"Watch websites for changes/updates" might be a start.