There are two main items to consider when writing a round: the lyrics
and the tune. Either one can make or break a round.
Lyrics
Good lyrics are certainly not a necessity, but they help. An amusing,
appealing, or insightful set of lyrics can turn a so-so round into a
favorite because people enjoy the words. Similarly, some rounds remain
unsung because the words are uninteresting, incomprehensible, or difficult
to sing/remember.
Tune
You can write a round that sounds fabulous but is horrible to sing -- and
most of the fun of rounds is in the singing! Try to avoid writing rounds
where one line is obviously there just to round out the chords. If you
really need six C's in a row, spread them out between the voices.
If the tune sounds good by itself, your round will be more memorable.
Also, check how it sounds when sung with different numbers of parts --
sometimes there aren't enough people available to sing all the parts.
To be expanded and added to in my copious spare time...
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