Amazon Reviews vs. Goodreads Reviews
Both review systems are owned by Amazon, but they have different methodologies behind how they work. As a result, they can end up being drastically different for novels that are up for sale.
Goodreads is a community of readers who recommend books to one another in a viral system where certain books are able to climb to the top. Amazon, on the other hand, is a storefront that actually recommends the buying and selling of books. They are actively trying to promote books to readers for purchase, not by quality of the books, which means that sometimes they can promote subpar books simply because the author is popular or because it is already selling well.
Goodreads doesn't suffer from this problem. The community is simply interested in talking about books, their virtues and failings, and working together to pair the right book with the right reader.
Amazon has a system in place whereby anyone who wants to rate a product also has to write a review to justify their rating. This, in turn, thins down the number of reviews dramatically and it is not uncommon to see tremendously more ratings on Goodreads than might be found on Amazon. That being said, Goodreads suffers from a completely different problem where a lot of readers don't actually utilize the rating system for ratings. In fact, for many people the rating system is used to denote how far along in a book someone is, or which books they want to read next or down the line, or any other number of things that have nothing to do with rating a book.
On top of that, many people judge a book harsher on Goodreads than on Amazon, or will simply click down a list of books with low ratings because it is easy and very inconsequential. All in all, Amazon's system of restricting people to only rate if they also review thins down on the randomness of the system.
Alternatively, Amazon also suffers from the ongoing plague of issues related to review purchasing and other things, but to think that Goodreads doesn't suffer from this would be wrong. The only difference is that Goodreads doesn't really stop this practice because it isn't selling books, only recommending them, so the stakes aren't as high. In this case, the only difference is that Amazon is actively striving to fix this issue and has made great gains recently.
This one is easy: Amazon reviews are linked up with a sales page and reflect in the Amazon algorithm to recommend your book to new readers. Goodreads has an internal viral networking system where it tries to keep readers on the Goodreads site. The site itself will recommend books for readers, but this isn't directly linked to reviews.
Both sites will earn negative reviews, though as I mentioned above Goodreads typically trends a lot worse than Amazon.com. That being said, Amazon negative reviews are far more detrimental than Goodreads ones, but in both cases there is nothing wrong with having negative reviews. Not everyone will like a book, and something with only positive reviews can actually make people suspicious.
Goodreads is a much better community than Amazon, but all in all the reviews on Amazon are worth more in the long run for an authors career. Goodreads reviews will trickle in overtime, and generally there will be more of them because it is so easy to just click/rate a book, but the extra effort a reader needs to put in on Amazon makes those reviews more meaningful.