Some other things that can fail:
oeb-page-head and oeb-page-foot: I know of no reader that supports them properly, but at least they should be removed from the text flow, i.e., treat them as "display: none".
SVG support.
Proper PNG support: at least ADE seems to have problems with PNG images with less than 8 bit colour depth.
Hyperlinks in images: Just make a <a href="..."><img ...></a>, and it doesn't work in ADE (or not always). Not to mention imagemaps.
About the test file:
The ligatures test depends on the font. If the used (default or embedded) font has no ligatures, not using them is not a problem. If it does, it can be considered so.
Being picky, the smallcaps test should have the spaces outside the <small> markup
