What makes a well-written fanfic?

[SonicFan47]

Title.

Seriously, what do you guys think?
For any fanfic, not just Sonic.
I think that one must know what they are talking about. It kinda bugs me when a character's...um...character is twisted around for no reason.

Anybody else?
(This is the part where you guys (and girls) respond) .

[Elija2]

A well-written fanfic obviously has to be well-written. There can't be errors (grammar, punctuation) and it can't be lazily done. Also, you shouldn't jump from scene-to-scene very quickly, as this can confuse the reader. It should also have a good story, not a dull one mainly focused on action. It's also best not to use fan-characters as most readers will not be able to relate to them as they would an actual character that they already know well. It is okay to add in a fan-character as long as you focus good on his/her background and purpose. And finally, add in as much detail as possible.

[SonicFan47]

Adding to your statement, Elija, I also think that a well-written fanfic doesn't have...how do I put this...unneccesary(?) language.
What I mean by that is if you have a character who's personality would indicate that they use certain words frequently, then by all means, have them curse regularly, but don't make it overkill.
i.e. Don't make your standard villain be F-in' around in every sentence.
Words can have a lot of power, and (typically) with your standard swearing terms, less is more.
You also don't want to have an access of boring "talking" scenes in which the characters do nothing but talk.
Throw a little tension in there! I'm not saying that there should be zombies bursting through walls after ever character finishes a thought (unless it's a zombie story, and even then that's pushing it), but don't "Charlie Brown" it, where any line of dialogue could be replaced with "blah blah blah".

[Dracoslythe]

I myself, believe that a good fanfic author has a FIRM GRASP on the subject that they are writing about.
Take someone like myself, who loves, say...well, let's use a real example.  I love Touhou.  But I don't know the story through and through enough to be able to write a fanfic on it, so that fanfiction would suck so badly.
But take something that I know better than the back of my hand, such as Inuyasha.  I could write for a year, and still have something different to write about(well, used to.  I pretty much forgot most about Inuyasha) because I knew every detail from where/when/how the arrow hit/killed Kikyou to every character's name/species/power, to even knowing the Japanese names for the attacks and shit.

Like Eli said, solid scene transitions, solid plot/story line, and SOLID SOLID detail.
Not trying to boast or anything, but if you're a fanfic/story writer, I'd like you to do a little exercise.  In 8th grade, I got points taken off a writing assignment for going too in depth with detail.  I'd like you to take a simple scene (such as a meadow in spring) and exaggerate the details on it, making it into something that someone would be overwhelmed with the detail.  Down to the flowers and the blades of grass.  That was a simple school assignment, but the more detail you put into fanfics, the more vivid that your audience's mental images are going to be.

And to elaborate even further to SF47's  post, unless the characters are supposed to be all standing around talking (as in that there's nothing to do, and it's casual conversation), then by all means, but even then, try to make your audience FEEL the emotion that's between the conversation partners.  But if it's like an action scene, throw in some action WHILE the characters are talking, like so (bear with me, I'm coming up with this as I type)

Selena's face twisted up with distress as Raiyn swiftly swept his foot under her.
"What is your problem?"  Selena cried to her almost alienated brother as he took another dust-stirring swing with his foot.  While holding her distressed expression, she looked at Raiyn, square in his hollow green eyes.  All she could do as he stared blankly back at her was let out a quick gasp, and with that, she found her body limp on the dusty ground.
"There is no problem, sister," Raiyn very slowly said, with an empty, monotone voice.  "It's just you.  You're not my sister anymore."
"RAIYN!"  A shrill scream soon filled the open area and echoed around.  "I-it's like you aren't human anymore...I..."  a small tear filled the young woman's eye.  "I...want my brother back."

Now compare that to this, which is pretty much the same as above, only without the extra spice to it, but as SF47 puts it - "A boring 'talking' scene in which the characters do nothing but talk."

"What is your problem?"  Selena said to her brother, Raiyn.
"There is no problem, sister," Raiyn said.  "It's just you.  You're not my sister anymore."
"RAIYN!"  Selena screamed.
"I-it's like you aren't human anymore.  I...want my brother back."

See the HUGE difference?