Perhaps not as grievous, but a noticable omission as well, are
ligatures for "fi", "fl", "ffi", and "ffl", etc.
Unlike em-dash and "ae" and "oe", the "fi" and similar ligatures are
presentation forms. Some fonts have them, some don't, and their usage
is up to the presentation engine.
They are presentation forms because whenever the two-glyph sequence is
correct, the one-glyph sequence is correct (OK, perfectionists might
object to a ligature in "leaflet" and "cufflink").
This is different from the case with "ae", "oe", etc., where the
ligature is a matter of spelling (based on etymology). The ligature
is correct in "encyclopaedia", "aesthetic", "amoeba", "Oedipus", and
"phoenix", but not in "aerosol", "aloe", "shoe", "poet", "Kafkaesque",
"maestro", "whatsoever", or "Michael". And in German words like
"Schroedinger" and "Jaeger", the digraph represents a character with
umlaut (so in fact the two-glyph sequence is actually a presentation
form of a single character).
Conclusion: the ligatures ae and oe and the characters a-umlaut and
o-umlaut are characters, not glyphs, and belong in the character set.
The ligatures fi etc. are glyphs (presentation forms), and do not
belong in the character set.
-s