My favourite forum section has changed over the years, it used to be the device specific one, especially when I was looking to buy my first ereader. However, now it's probably the one I post in the least, the "deals, freebies and resources". I've grabbed several bargains I'd likely have otherwise missed due to members posting in there. A close second would be the news and general discussions because you can frequently find interesting debates going on that on the whole remain debates and don't (often) descend into flame fests.
However, if you want the favourite part of the site as a whole, it's the mobile read library. It's clear several members have spent a great deal of time and effort putting it together and formatting books for it.
Improvements wise, I'm not really sure. I guess the biggest hurdle is knowing that a book in the library is worth reading and really that's covered by the topics going on in the reading recommendations forum and the book clubs sub forum which I've only just realised exists
I knew about the book club thread some time ago, didn't realise there was now a dedicated forum.