I still think that the SD slot should be returned in future models. Back in the old days of reading on my Clie I would use memory sticks to arrange my library by subject. Nowadays with Dropbox my library is available to all my PCs and my mobile phone but *not* my Kindle. My Calibre folder is nearly 10GB (I've purchased a lot of books and also have many more free books in Calibre) so carrying my entire library on a portable dedicated reader will require a bit more memory.
PDFs are much larger than other ebook formats and I know that my DX fills up fast if I'm loading PDFs. I always thought it was a peculiar choice by Amazon not to include an SD slot on the DX when it supported PDF.