These would be my suggestions for publishers, based on what I have learned from fixing problems in the ebooks I have bought:
Most important, test the book on a Kobo device after you have uploaded it:
* Check that the Font Face, Font Size, Line Spacing, Margins, and Justification can all be adjusted, at least for the main body of text.
* Check that objects which are supposed to be right- or centre-aligned (such as headings, scene break asterisms, images, etc.) remain so when the Justification button is set to Left or Full justification.
* Check that the progress indicators (TOC chapter mark, page number, chapter graph and stats) work correctly as you advance through the book.
* Check that images are displayed at the appropriate size relative to the screen size, and that images which are supposed to change size as the font size changes do so.
To create a book where all of the above work correctly:
* Don't set the font-family, line-height, or text-align styles at paragraph level for the main body of text. Instead set the defaults at body level and let the paragraph styles for the main body of text inherit from there.
* Don't add any left/right margins or padding styles to <body>.
* Don't use <p> or <div> for titles, headings or other text objects which are supposed to be centre-aligned or retain a fixed alignment. Use <h1>, <h2>, etc. or <blockquote> etc. instead.
* Don't use units of px, pt, in, cm, etc. for anything unless you are certain that it is really necessary (it almost never is). Use em or % instead.
* Make sure the items in the NCX toc are in exactly the same order as they appear in the OPF spine.
* Don't use bitmaps as a substitute for glyphs which might be missing from some fonts (such as Hebrew or Cyrillic glyphs in an otherwise Latin text.) Use an embedded font which contains the appropriate glyphs instead.
In addition, to improve the reading experience:
* Make sure line drawings or other images that are supposed to be in one colour, such as those created by scanning a black-and-white drawing from a book, really are black-and-white rather than greyscale.
* Make sure that every chapter is in a separate file. Keep the files as short as possible. Consider breaking files larger than about 100KB into seperate files even if it means splitting a large chapter between files.
* Don't create scene breaks simply by adding a margin-top style to the opening paragraph of the scene. If you really must indicate scene breaks just with whitespace, then either use a padding-top style, or add a blank paragraph <p> </p>to the text instead.
* Don't use breakable elipses like ". . .", use "..." or ". . ." instead.
* Think about what your book will look like in a few years time when ereaders are 600dpi or higher. Better to make bitmap images too big and have to scale down rather than too small and have to scale up, and best to use SVG instead of bitmaps if at all possible.
* Use EPUB2 format, unless EPUB3 is really necessary. (It almost never is.)
Metadata:
* Don't include the series name in the title or subtitle fields. Put it in the series field.
* Don't list the book under inappropriate genre subcategories. Listing
Sherlock Holmes in the Space Opera subcategory is just a form of spam.