most important:
probably a reasonably well-implemented game, that also has good gameplay.
basically, if the game is buggy/broken/... this isn't really good.
nor is having things sufficiently kludged and brittle that things barely work as-is.
programming and artwork tend to take up a big chunk of the time IME (though in my case, programming tends to dominate).
the main thing that is lacking:
that the gameplay doesn't suck...
but, it is sort of a mystery of "how will I best go about making things better?...".
basically, it is a problem of sucky gameplay and boring worlds...