@keobooks My impression at this point:
Readernaut is more well-designed and has better groupings for the books. Goodreads has Read, Currently Reading, and To Read. Readernaut has Finished, Currently Reading, Plan to Read, Abandoned, Reference, and Wishlist. I wish that Goodreads also had “Abandoned” and “Reference” included as options in the default/required shelves.
Goodreads seems more feature-rich (though there are a couple of omissions of features that Readernaut does provide, such as the ability to correct the number of pages in a given book, which is almost always wrong in such databases and really comes into play when you’re tracking your by page). Goodreads also appears to have a more active community.
But I realize you’re mostly talking about writing reviews. I only ocassionally write reviews and am more interested in the other aspects of these sites (tracking what I read, seeing what other people are reading, and discussing what we’ve read or want to read).
Addendum: Perhaps the most disappointing thing about Readernaut (other than the fact that no one else seems to have adopted it) is that it still lacks Facebook and iPhone apps. I wouldn’t ever use the Facebook app, but I think that having one would lead to increased adoption. I know far too many people who won’t do anything online unless they can do it through Facebook. An iPhone app would reduce friction in my own use of the service, i.e., I wouldn’t have to open my computer every time I want to update my progress or add a book which would mean that I could spend more time actually reading and less time getting distracted online.