I believe we not only have to consider quality but also qualities: not all the qualities that make a great short story are required for a great novel and vice versa.
Different readers, and writers, prefer different qualities. Some really like short fiction, others love massive doorstops. I like both. I thoroughly enjoy magazine short fiction (I've read the entire decade of the 1950's for Astounding (Analog), Galaxy, and If) but I also love long epics like Lord of the Rings.
One thing I've noticed is that different genres seem to lend themselves to different lengths, too. I've read far more good short science fiction and horror stories than fantasy. There's something about the fantasy genre that works best for me in a minimum of novella length.
RHadin made an interesting point about character, saying that short fiction wasn't really long enough to really get to know a character. This may be one reason why SF has often worked better in the shorter lengths. Real SF (as opposed to the pulp adventures in space the genre started with) is a literature of ideas, and many of those ideas can be stated in a single sentence.
It's a lot easier to write a good story about a one-sentence idea than about a deep character. However, if a reader doesn't really care about the ideas as much as the characters it probably won't appeal to them.
Different strokes for different folks.
Not even a perfect example of something they don't like will appeal to someone. I don't like mushrooms but love onions; I'm enough of a cook to tell the difference between a perfectly cooked mushroom dish and a bad one, but I'm still not going to eat it. I'm going to have my onions.
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