While there are some intermediate forms, as I understand it what makes
operetta distinct from opera is some combination of the following:

1.  A 'light' plot.  If not actually comic, it deals with ordinary people
in ordinary life-crises; opera, on the other hand, deals with great Kings
& Queens, Gods, Heroes & Heroines and huge, heaven-storming emotions.

2.  A great deal of spoken dialog.  Opera has little or none.

3.  Highly melodic music.  The songs should be accessible--preferably
hummable or whistleable (sp.?) by a person with minimal musical training.

4.  Natural and unaffected vocal technique, as opposed to the highly
stylized vocal technique often required by opera.  Singers with minimal
formal training--although not minimal talent or musicianship--can often
sing operetta, while such singers would need much additional training to
tackle most opera.

5.  Lack of extremes, both in emotional content and musical qualities.
(Not extremely loud or soft, not using the extremes of the singers'
ranges, etc.)

6.  Operetta does not take itself seriously; opera does.

As I said, there are some works considered firmly in the 'opera' category
which have one or more of the above criteria.  La Boheme, for example,
deals with ordinary people in ordinary situations, Magic Flute has a great
deal of spoken dialog, most Rossini is quite hummable or whistleable, etc.
But if (say) three of the five are present it is probably an operetta.

The next question is "Where does one draw the line between operetta and
musical comedy?"

Bruce Alan Wilson