What is "feedback"?

Simply put, feedback is the analysis of a given piece of writing. More formally known as a critique, the object of the exercise is not to render a simple statement (good / bad / indifferent) but to evaluate the work more closely.

Factors to be considered for specific feedback depend largely on what a particular fanfic is trying to achieve--if a story is set deep in its own alternate universe, then there will be little if any point in comparing the depiction of specific characters with those portrayed on the TV show in question. Even then, however, the various reasons for character adjustment must somehow be explained--even if by a single sentence saying something like, "Five years in the future, everyone is dead except Xander, who has suffered brain damage." Here are some examples of things we look at:

1) Concept - originality and exploitation
We've all read the fanfics where the characters become totally inebriated and watch movies at a sleep over, then play truth or dare. This is little more than an excuse for some uninhibited NC-17 action, and is generally recognized as such; still, just because a story has a common premise doesn't mean it can't be good in both the technical and fun-to-read senses of the word--in fact, one of the best fanfics I've ever read has been a ‘truth or dare' story. Originality is important, but of equal importance is how well a given idea (original or not) is used by the author. Hackneyed ideas are re-used constantly in both the amateur and the professional writing worlds (for example, anyone who thought "Star Wars" had any new material in it, might want to read Joseph Campbell's "The Hero with 1,000 Faces"). Always, without exception, it's the way a concept is used that makes the crucial difference.

2) Story structure - the actual plot
Was the story set up clearly? Did it progress in a rational fashion? Did it reach a conclusion rather than simply stopping? These questions are more than just important, because how and why the characters get from A to B--and then on to C, or perhaps back to A--are the bare but absolutely essential bones of a story.

There must be a reason why the characters do what they do, and their actions must have reasonable consequences. This mandate is more or less relevant depending upon the category of the story, but even the smallest vignette of personal introspection must have some form of internal structure along which it advances to a conclusion.

3) Characterisation - do the characters talk/think/react as they should?
This can work on many levels, but the primary categories are the ‘voice' of the characters and the (internally justified) basis of their motivations. The first is relatively easy; we all watch the shows, we can all imagine the characters saying the lines, and we know whether or not those lines sit comfortably with them. The motivation for their actions, however, are always going to be a delicate subject; each viewer can and does read a wide variety of motivations into a given character. For example, in Roswell some believe Isabel's only romantic interest is in Alex, while others believe she's panting for Michael--while in Buffy, some believe that Spike is a vicious, cold-blooded killer even as others see in him a capacity for human love. None of these views are "right" or "wrong" in any definitive, all-encompassing sense. All fanfic is fantasy to some extent, and a lot of it works on the premise that characters feel a certain way about each other that the series does not itself acknowledge. Nevertheless, there must be some resemblance to the original for a story to be fanfic as opposed to merely original 'fic' with familiar names.

4) Writing style
We're not your English teachers, we're not going to scribble over your fics with a red marker pen, but writing style is important. At its best it allows the reader to become completely immersed in the world while the story plays out in her head. At its worst it makes reading becomes a struggle to extract the sense from unwieldy text. We're not going lecture you on basic semantic and grammatical "do's" and "do not do's" but if a fic is badly written we will try and give a few general suggestions on how to make it better.

5) Technicalities
Mistakes as to the small facts of the characters lives just happen. What was Buffy's address? Which finger does Michael wear his ring on? Sometimes they're irrelevant, sometimes they're not. While the readers aren't show-encyclopedias we will mention it if we spot a discrepancy. A word of warning to the readers out there, though, the only thing more embarrassing than being a writer who's made a silly error, is the being the reader who called her on it and was wrong.

6) 'Twinge' factor
I found this at the Critical Mass Blue website and it's a great medium through which can be expressed all those niggly nebulous concerns that can't quite be put into words. I'll let the webmistress of the aforementioned site explain:

7) Specific grammar, vocabulary, sentence structure etc.
Considering each word and line in turn is the role of a beta-reader (and an extremely thorough one at that), it's far too detailed for feedback.