I thought it would be good to share examples of well designed EPUBs.
There are some great examples of what can be done with CSS at
http://epubzengarden.com but I haven't seen these on an ereader to confirm they look good on a device.
There are a few basic standards that EPUBs should meet, outlined clearly here:
http://www.publishinginnovationawards.com/the-qed
By sharing examples of well designed EPUBs, hopefully we can see a benchmark emerge.
NB: I don't have anything to share myself at this point. Hopefully more experienced ereaders/designers can get things started.