Amazon have a terrible reputation when it comes to user interfaces.
There are some amazing things about Amazon. I use Amazon often. But the website sucks; search is pretty much broken (it's like web search used to be); Kindle ebooks sometimes have laughably terrible typography.
Similarly, I don’t know anyone who is very confused by Amazon property IMDB.
Amazon.com search may not be as good as Google’s, but it isn’t the worst thing ever. I will agree they still need to work on relatively basic things like spelling correction. But remember A9? Amazon have certainly invested in search. And certainly their book search is worlds better than GoodReads’ is! Basic things like "hitch-hiker's" vs "hitchhickers", IIRC, have tripped up GoodReads search, but don’t make as much a difference on Amazon.
(Should also mention that the interface I am discussing is the one with which I’m familiar: their current iPhone app as-of 2012–2013.)
There is also no capability to discuss only up to a particular portion of a book or any easy way to read books together etc.
The product has a lot of potential, but there are some fundamental usability issues holding it back imo.