On Sun, 25 Feb 1996, Jon K. Hayashi wrote:

What qualities make a good fanfic?  

That's easy.  If you like it.

What qualities make a bad fanfic?

If you don't like it.

Is there any sort of quantifiable that beginning writers can use or is
it simply "I liked that one."?  How much of it is mechanics vs. type of
story vs. genre? 

Mechanics makes a story difficult to read.  Spelling, bad grammer, poor 
line wrap, bad punctuation... etc...  These make it very hard to read, 
thus a lower opinion.

mechanics = continuity, writing style, etc.

Writing style shouldn't matter too much, but I prefer to read prose.  
However, some stories are better in script format.  Continuity is nice, 
but can really get in the way.

type of story = crossover, continuation, lemon, etc.

Naturally, lemons are subjective.  But as for the rest, it's very much 
like the continuity issue.  Crossovers have the problem of too many 
characters.  I've run into this, so I dropped a bunch of Ranma 
characters from mine.  Basically, no one you haven't already seen is 
going to show up in mine.  Continuation depends on if the story is 
finished.  If it is, continuity becomes a problem.  If it isn't, it 
already was a problem.

genre = Ranma, AMG, BGC, etc.

That is utterly personal taste.  Which reminds me...  has anyone seen a 
Nuku-Nuku fic before?