I felt like I have to write something, then I browsed through the archives
and read the whole topic.  A few good points were in some posts.

First and most important.  Operetta is not easier (from performer's point
of view).  Some people still think that way (hopefully not here).  Just
listen to Elizabeth Schwarzkopf recordings, or Rene Kollo.  Somebody wrote
that Die Fledermaus is not best example of the operetta.  What is then?
Strauss wrote only operetta's - the works are all based on the same model.
Many are really difficult.  Not only the Adela part.  Vienna Blood is
considered by the conductors as the hardest to learn and perform.  Zeller's
"Vogelhandler" has so much difficulties for the singers and the orchestra,
take "Wilya" from Widow, I could make that list much longer.....

The difference lies much deeper - I think.  The form of musical numbers
is usually "classical" - maybe even mostly.  It is a sonata form, rondo,
aba or other quite simple form with very often one theme.  It is not a
rule, but may be helpful in deciding what is operetta and what is opera.

The operetta uses mostly a small - again - "classical" orchestra.  Very
seldom instruments like english horn, contrabassoon etc.  Not always even
double woodwinds.

I also think that operetta is in some way connected to opera buffa.

Every music depends only on the way you perform it.

Tomasz Biernacki
conductor http://www.dyrygent.prv.pl/
Poland