Fidelity to the original characters and setting is good - and by this, I include credible explanations and explorations of why things are different in your fic, if they are. You have to remember that your work came from something, and what its relation to it is, or else wash your hands of it all and write original work instead.
Likewise, since you're doing a derivative work, creativity is especially important. You need to give people a good reason to read your fic, rather than just revisiting what exists already in the official story. Continuations should move the setting and cast in interesting directions. Adaptations to another medium should exploit that medium's benefits in creative ways - for instance, using the greater depth allowed by literature to flesh out characters and settings in intriguing manners. "What ifs" should strive to answer the question in ways that are unique and believable. Crossovers should create something greater than the sum of their parts.
In short, aim for excellence, whilst still keeping an eye on the story's roots.
I think there's a difference between an acceptable anda good fanfic. Acceptable is having all the characters in character, plausable plot, etc, all the things you should expect a fanfic to be, whereas agood fanfic is described expertly in post 2. (Btw, on mobile again, apologises on anything i missed)
I think a good fanfic, mearly uses the original as a starting point, and takes that to explore questions that weren't addressed in the original. It could be, what if these two fell in love, or looking deeper into a character's past. Other good fanfics will take the characters and place them in a completely different setting, and will keep them true to the original while they branch off into strange situations. One of my favorite stories took the original fandom, and put it into something much bigger. He combined both original in cannon in such and incredible way.
>>I agree with all the above, Iaculus actually said pretty much all!
I want to add also, as long as the writer knows their fandom well, and the character's they use, and keep them as true as possible, into whatever kind of story AU they put them, then the story will be good.
This is for any fiction: your piece should elicit an emotion or feeling other than boredom. That's why My Immortal is better than Twilight, whether you laughed or cried as you read it.
You should also try and make a piece that people who don't know the fandom will enjoy (ie: I'm currently reading F&L's Mulan fic even though I hate Disney). If you're doing a crossover, explain who everyone is, and don't neglect one side. Generally, crossovers are AU anyway, so it makes sense to explain who the characters are in terms of your unique universe. Karanguni and Ellnyx's FF7/FF12 crossover is a great example of this. I'd say that using 3rd person limited perspective, or even 1st, makes the most sense for this, to avoid telling instead of showing.
Much of the rest has already been covered. I'll add things as I think of them.
>>6 sure changes can be made :)
if the writer really knows the character they will know what changes to make so they won't be OOC :)
and well it works more easily for characters who are not really open in the original story.
for example in my fandom Haruka and Michiru.
Haruka is a character that is really easy to work on and develop her and change her without making her a major OOC. because she is really a close character.
Michiru on the other hand is an obvious character, you can't change much about her.
I agree with everything Iaculus said.
I'd also like to add that it should read like published work, by being correctly formatted, with good spelling, grammar and punctuation. The summary should make you want to read it without giving too much away or being too vague.
I hate it when good story ideas are marred by an author who refuses to use a spell-check and/or beta. It doesn't matter how in character you write your fic, if it has so many spelling, grammar and punctuation errors that it throws the reader out of the fic, it is not good.
I had to stop reading a fic which had the very stortyline I'd been looking for merely because of they way the author used thoughts. It was very off putting to have the POV switch to whoever was speaking in the same chapter.
It was a case of..
"Why are you standing so close to her?" A said (She's mine, damn it!)
"No, reason A." B said (Because I love her you dope, is it not obvious?)
This isn't a direct quote, but it was the way the author was writing.
As fanfic writers it is tempting to try and tell the reader what each character is thinking, but it can get annoying and confusing changing POV all the time. A good author shows other character's thoughts and feelings through their actions and words. It is hinted not specifically said.
A 'good' fanfic is determined by the reader, rather than the writer. However, there are general rules that can be applied to make a fanfic appealing to the reader.
First is the POV. In 1stP, choose a character, and stick with them. Changing is off-putting. In 3rdP, make it clear when you switch to other characters.
Second is simply grammar and sentence structure. Making it clear and easy to read adds to the enjoyment.
Third is to try and stay true to the original work your story takes after. Characters going OOC are a common problem in early stages, potentially ruining the experience. Unless, of course, the OOC is a result of something earlier in the story, and the character has progressed and changed that way.
Finally is to have a clear destination at the end of the story, and move your characters toward that goal. Don't go off in another direction, and come back to the original goal later. Unless, of course, the original work you are fanfic-ing demands such action. Too many other goals can confuse a reader.
An acceptable piece of fanfic has minimal spelling or grammar issues, a purpose behind the telling and the characters are IC.
A good piece of fanfic has all these things, with the added bonus of a believable and considered plot and being a good read. The kind of thing that makes you want to read more and pleased to see an update, or to keep an eye out for the author.
And exceptional piece of fanfic stays with the reader long after they finish reading and sees the characters react to developments and be believably changed by them.
In my experience, acceptable fanfic is the largest percentage, larger than badfic because most people put at least a minimal amount of thought into something they share with the public. Good fanfic frequently depends on the individual reader and what they like to see. Exceptional is rare and that's probably a good thing, otherwise we'd all be stuck at our computers 20 hours a day.
A story which has only a few, if any, grammer or spelling problems. Also, the characters have to be recognisable as the characters from the book/TV show and not just random people with their names attatched. The characters shouldn't be OOC unless there is a valid reason and the whole story is an AU story with a good plot line. I like to know that the author has taken the time to create the story and hasn't just written the first thing that comes into their head and posted it.
Well A good fanfic. for me should have:
Those are my sacred SEVEN PILLARS OF FAN-FICTION. Is an open list, though.
As for cross-overs.
These are my CROSS-OVER'S ESSENTIAL THREE. Of course, this is another open list.