I use Fresh and Clean on my personal blog and see that you have figured out how to have the featured image thumbnails appear on the front page of the blog, and conswquently, on Categories pages too.
Category/archive pages: full posts or not
Unless you use the P2 theme which is a Twitter-like theme, the front pages of our blogs showcase content and comments are collapsed on that page. You cannot change this as the way the comments function is coded into the core of WordPress. By clicking the post title or the comments link the posts on the front page open on their own page where all comments display.
In a new blog one does not have many posts or comments. In an established blog if the comments did not collapse on the front page it would become a mile long in no time flat and page loading time is a page ranking factor. We have only seconds before visitors refuse to wait and click out.