You probably already know this, but for me the best way to find new science fiction is through the Baen web site at:
http://www.webscription.net/
Now, back to your original query. I agree that I have yet to see a web store with a good way to casually browse any non-trivial collection. However, Amazon does have two features that I find extremely useful.
Go to a book that you know you like on Amazon. Then look for two areas: 1) Customers Who Bought This Item Also Bought; and 2) What Do Customers Ultimately Buy After Viewing This Item. These should give you lists of similar books that you can choose from. You can even 'chain' these recommendations by clicking one of the recommended books and then seeing what is recommended on its page.
In some ways this is much better than what you originally requested since it can lead you through a similar genre quite efficiently. But I do admit it is not perfect. Books won't show up on the list until they are popular. I can see this approach leading to an insular focus on the most popular authors and freezing out new authors who are trying to bring a fresh view to the genre.