I'll be honest... from my preliminary (and very cursory) experimentation, the only real difference here is that kindlegen (and KindlePreviewer) v2.8 is
accepting ePub3 as input. But it doesn't really appear to be
creating KF8 files that are significantly different than what the previous versions of Kindlegen created. I've not really tested any of the multi-media stuff (so take what follows with a grain of salt), but there doesn't seem to be any real
inherent advantage to starting with an ePub3 document over ePub2 if you're not ready to make that particular leap with your text-based ebooks.
The only thing that
may be an advantage to some is that with ePub3, you don't have to mess with creating an NCX if you don't want to. Kindlegen will automatically create an NCX from the Navigation Document.
Here's a small sample of an ePub(3) document that KindlePreviewer doesn't bat an eye at. It's nothing fancy, but it shows an example of the Nav document and an ePub3 OPF file if you're interested. Just don't open the epub in Sigil though, or it won't likely
stay an ePub3 ebook.
EDIT: The method 2 for cover images matches exactly what's been in previous editions of the Guidelines, so far as I can tell. The name values may have changed, but it's the same thing: a meta tag with the reserved "cover" name and then a manifested item using the meta tag's "content" property for that item's "id." Same-same.