Unfurl uses the target's HTML code for og:description. If the page you're linking doesn;t have this, which Amazon.com doesn't appear to, then I would suspect that XF is failing back to a very basic link.
There was a thread I started recently - xenforo.com search isn't working right now so I cannot locate it - about unfurl with IMDB, and XF response was that they aren't going to improve/change unfurl support if the target site being linked doesn't support Open Graph appropriately.
It appears amazon must have made some change. I tried this on a stock skin and with add-ons disabled and the unfurl doesn't work. Testing it shows that Amazon isn't sending back the proper data.
I enjoy the XF unfurl feature for the most part. External links usually look nice in posts when it works. External links look fine (raw URL) when it doesn't.
However, there is one domain that vexes me terribly - msn.com. All links to news items on msn.com "unfurl" to a simple link with "MSN" as the anchor text. Those links look horrible in the forum and are less informative than having the actual URL displayed.
I would absolutely love it if the XF ACP allowed us to define a list of domains that should be ignored by the unfurl feature so I could add msn.com to the list and never...
yeah amazon can probably only be solved using their API or something. i remember wordpress used to have some addons, paid probably, that generated a very nice product box for blog posts.