Things in Ranma fanfiction that really annoy me in no particular order:

• Removing all character flaws without having the characters work to improve themselves. Ex. Akane or, even worse, Nabiki suddenly being great perfect people.
• Ranma and chosen girl/s get together and it solves all relationship problems. Yeah right, Shampoo/Ukyo/Kodachi are going to give up just because Ranma married Akane/someone, does anyone see that as being the least bit believable? If anything things would get worse not better. Leaving out the other girls, things between Ranma and the girl won't automatically get better or be perfect. If Ranma ends up with multiple girls (wouldn't rule it out considering Ranma's life), the girls would still argue with one another and maybe team up against another girl or Ranma.
  
• Establishing Ranma to be weaker/less powerful/etc., than his canon self and then giving Ranma a power boost that is below his canon abilities. Example Ranma is said to get much stronger through some means and can now lift a ton, Ranma in manga has lifted and even threw things that weighed more than that. This is just dumb to me, it shows that you either didn't know Ranma's abilities or you wanted to power up Ranma without powering up Ranma
• Drastically dumbing a character down or increasing knowledge. For instance I've seen a fic in which Ranma didn't know basic addition as well as fics where Ranma knew advanced physics. Making Ranma knowledgeable in something not shown in the manga is all right but explain it in a believable way. For instance if you need Ranma to be knowledgeable in physics explain that his junior high science teacher showed him how knowing physics benefits his martial arts or something similar (There is nothing in manga that says he is not knowledgeable in physics). Whatever you do, do not use the excuse Ranma was pretending to be dumb, that is out of character for Ranma and don't make Ranma knowledgeable in things that he is shown to not be knowledgeable in (like Shakespeare and how the Jusenkyo Curse really works) or Akane suddenly knowing how to swim, cook, sew, etc.
  
• Completely ignore established power scales, e.g. Ranma defeating Happosai without trickery/traps/etc., though if it's a crossover/fusion it doesn't bother as much since a balance state has to be achieved to tell the story. Example: if DBZ fusion ok to pump Ranma's abilities up to what is shown as the max for Humans from the get go just mention in an author's note or something that your doing that to balance the abilities and that is where Ranma would be had he been in the Dragonball universe - not to the level of Sayians.
• Ranma with a guy. This can be done but is very difficult to do without me hating it.
• Out of Character, unless it is explained as to why they are out of character this shouldn't be done, though realistically, since the stories are not being written by the original author, some ooc is unavoidable.
  
• Having Ranma only have normal human abilities or pointlessly lowering his abilities. I really hate seeing things like Ranma being injured by a normal car. Though this can be done with out bothering me so long as it is done to all the characters or is required for the story (ex. of required Ranma/ Buffy crossover manga Ranma could defeat practically every villain shown with relative ease so his abilities i.e. strength, speed, etc., should be lowered until that is no longer the case but not so much so that he is the weakest thing there don't forget to take in the fact he does have many years of martial arts training which should be taken into consideration).
• Super powered/Godlike Ranma. Truthfully, This doesn't bother me because in my opinion manga Ranma is already like that in many ways. He's a mid-teen and can hold his own against people with decades, possibly centuries, of experience, learns techniques at insane speeds (that surprise those people with those years of experience), is good at practically everything he tries to do (can cook, clean, sew, do repair work, martial arts, etc.) etc. The reason it's on this list is because a lot of the time the godlike is completely pointless serving no purpose other than making Ranma into a god-like being or is used to make some other character seem pathetic in comparison. If you do decide to make Ranma god-like, make damn sure you don't miraculously remove his faults in the process. Look at mythological gods they generally aren't faultless.
  
• Crappy explanations as to why Ranma left Nerima or how he got rid of his other fiancees. If you can't think of or don't want to explain why Ranma left Nerima, or how Ranma ended up with so and so, don't bother doing so. Just start your fic wherever you want to. It is much better to not explain it, leaving it to the reader to imagine that part or give little clues throughout the fic that basically tell nothing but give some idea as to what might have happened, than to make them read a few chapters of crappy forced reasons that are only there so that you can get to the point where you can start your story.
• I find it annoying for authors to tell at the very beginning of the fic who is going to be paired with who. If the match-up isn't important to the story it's okay to do this, but if it is the main draw of the fic, it is kind of taking away the drama. Do you really want to know before a story starts who is going to be paired up when there are several possible choices and that relationship is the main story line?
  
• Character bashing should be avoided. It's okay to show characters in a negative light but completely demonizing characters who do have some redeeming qualities should not be done.
• The excessive use of ya for you, ta for to, or truncating words. Hell the rendering of character's speech phonetically using nonstandard spellings in general (a bit is fine but too much is just annoying). They distract from the story (no one uses ya for you every single time). It is also not how Ranma talks in either the Viz or Japanese version of the manga, though the VIZ version does have him use some truncated words like didja but infrequently and generally when Ranma is angry (it is generally used in places where Ranma uses ruder language in the original Japanese). He was also not the only character to do so.