RE: ASIN and ISBN - I was torn on whether to include those, since I agree with you that it can be confusing. I also have the option of dropping the entire url in there and it will pick the ASIN out automatically. I figured that would be easiest for users.
RE: email - I didn't want to do the email thing because then I'd have to deal w/ unsubscribe and spam issues. I like making services that don't require a signup.
You could headline the box with "URL for the product or wishlist you want to follow", and then, in a smaller and greyer font below put "(you can also just put the ISBN or ASIN in there)".
I think their interface is a bit cleaner than RSSTalker and probably more developed right now.
Suggestions, feature requests, and bug reports are all welcome =)
RSS Talker is sweet. Matt, if you're reading this, and if you don't mind, here are a couple critiques:
The title of this page should really be the info in that bubble: "Track Amazon Price Changes With an RSS Feed". This should be the biggest, boldest font on the page.
I don't think the average web user is very familiar with the terms ASIN and ISBN. It could be as simple as "Enter your email to track a specific product on Amazon." The How It Works section can explain more.
Can you make it just an email service for people without RSS feeds?
This site has a ton of potential. It's a shame that Amazon's commission rate is so low. It would be awesome to open it up to any web retailer that is signed up through a service like Commission Junction. Some of them offer 15% commission. I'm sure you've thought through all of this.
Good luck!