Some behaviour that I have found differs between ereaders and am curious about:
-Open an ebook, read a few pages. Go to home page, side-load a bunch of new ebooks. How easy is it to get back to where you were reading?
-Does the ereader override font, justification, line spacing, margin widths, etc based on settings configured in the ereader if they are set in the ebook? You may need to locate a poorly formatted ePub as a testcase.