Personally I'm inclined to think that a fanfic is better when it
takes the character's established personality and behavior into 
account.  That doesn't mean not changing it.  After all, I wrote
a short piece in which Kuno was capable of introspection.  He was 
definitely NOT in character, even though I used this as a method
of exploring his established character by pretending it was reasonable
that the destruction of his family would make him a more sensible 
person.  Nabiki probably wasn't in character either when she 
marries him after his suicide attempt. Of course, I will mention that
the only time I've seen Nabiki do anything for anyone except her
father (and possibly her sisters) without asking for payment, was 
patching up Kuno after Ranma trashed him.  Even then, maybe she 
asked him to pay her after the scene was over and we couldn't see.  
But I did take their previous behaviors into account, and gave them
reasons to behave differently, and I do think that was better than
just casually ignoring their previous behavior.  

In the original
subject, Ranma was behaving a little oddly by insulting a girl he
doesn't know and has no reason to like or dislike.  Akane was
behaving far more oddly by showing Ranma nothing but indifference
even when his life was obviously about to be endangered, and by
not being angry with Ryouga when his pig curse was revealed.  This 
probably did weaken the story, not just because she was
out of character, but because it left her with no character to
speak of.  If Akane isn't a girl who cares deeply about most
things, what is she?  A bundle of conditioned reflexes trained
to hit Ranma when he talks or does anything.  

But OOC can be good.  For example, I now know that "Gatchaman" is the
story of a tragic figure forced into a life of suffering and evil, 
until eventually s/he is rescued by her supposed arch-enemy...

Or not.  Quite a bit of fan fiction on that subject or something close 
to it, though, and honestly I enjoy it more than I did actually reading 
Gatchaman scripts, which were often kinda cheesy although above average
for saturday morning cartoon fare.