Maybe someone should post a list of the things that seem to be taken for

 > granted as true and debunk them as either fanfic convention, anime vs
 > manga conflict, fanfic writer has too much testosterone at the moment...

Oh, I can have some fun with THESE. Let's see now......
  

 Nabiki doing any sort of adult business activity such as playing in the

stock

 market.  It's certainly possible, and maybe even suggested by the Kinnosuke
 story, but most people who use it just got it from another fanfic.

Actually, no. There are numerous subtle hints in both the anime and manga that
Nabiki handles a LOT of the families affairs: She planned a great deal of
Ranma and Akanes aborted wedding, and Soun/Genma probably wouldn't have
trusted that to just anyone.
  

 Shampoo constantly drugging food.

There's precedent in both the anime and mangs for this; the fanfic convention
is the KIND of drug used, specifically Passion Spice. A LOT of people think
that PS is a part of the actual story, but it's not.
  

 Ranma's mother as intelligent and perceptive.

This is an opinion statement; I personally think she's very perceptive. She
suspected the Ranko/Ranma connection LONG before it actually came out (kinda
like a Lois Lane/Clark Kent/ Superman thing going on) and actively tried to
catch them in the lie.

Evere see the scene where she clonks 'Ranko' on the head with a teakettle
because she thinks that'll turn her to Ranma? It's hilarious.
  

 Ranma's mother not knowing of the curse.

She DOESN'T. Not in the anime continuity, at least. As long as the author
doesn't specifically say 'manga universe' this is just fine. And even there,
she didn't find out until VERY late in the story; somewhere in the early 30's,
volume-wise.
  

 Ryouga constantly getting lost in Australia, India, New York, the planet
 Mars, alternate dimensions, etc.

I pointed this out about a two months ago. People insulted my intelligence and
said I didn't know what the hell I was talking about. It was very demeaning.
  

 The pool of drowned twins splitting Ranma into male and female.

This is a valid asumption, actually: the manga, at least, hinted VERY broadly
that this is what happens if someone who's already cursed gets hit with the
twins water.
  

 Mousse having to beat Ranma to get Shampoo's hand.  Or having to beat
 Shampoo.  Or variations with other Amazons and non-outsider males.

Dramatic liscence here. The rules of the Amazon tribe are never very clearly
spelled out beyond what happens to male and female outsiders, so you can go
nuts with your own interpretation.

It's a strict/loose interpretation kind of thing. 
  

 The dragon's hair still affecting Ranma after the Dragon's Hair story is
 over.  (This may be a little less common now, since the story has now been
 released in English.)

No arguement here. Although ignorance is a pretty good excuse.
  

 Tsubasa as a recurring character.  (He appeared in one manga story and

left.)

What's that got to do with anything? Just because he left doesn't mean he
can't come back. People bring Azusa and Mikado back all the time and they were
also just 'villains of the week.' Plus, there's precedent; Tsubasa shows up in
the "Nerimean Parade" that occurs in the first movie.

Although that's not the BEST example, since EVERYBODY showed in that scene.

 
 Cursed people not aging in cursed form.  (Re: Tarou)

This applies in Ranma's case. His curse was age specific; Spring of Drowned
GIRL. The curse will always give him the body of a girl. Not a woman, a girl.
There's a difference.

This applies ONLY to Ranma, though: All the other Springs mention nothing
about age, so the other's forms should grow old at a comparable rate with
their human counterparts.
  

 Tneshin Amaguriken as an attack: appears in anime, not in manga.

This is just flat-out wrong. The Chestnut Fist isn't grossly overused in the
manga, like it is in the anime (sometimes it seems like he's using it every
other episode) but he DOES use it. The fight with Ryoga right after he learned
Bakusai Tenketsu, for example. 
  
-Mercutio
"A plague 'a both your houses!"