Well hello. Interesting discussion you've got going on here. I have a whole slew of thoughts, philosophies and deconstructions of the fan fiction world, but I won't bore you with them here. Instead, I will offer a quick and dirty guide to my criteria for whether I read a fanfic or not. It all has to do with the author's stated (or sometimes unstated but still clear) reason for writing the fic in the first place.
"I just thought it would be cool."
I'm not reading it.
"I wanted to get (insert two characters who are probably of the same gender but are not homosexual as far as we know and have never shown any attraction to each other) together."
I might read it to see if it's worth bookmarking so I can show it to other people and we can laugh at it. It's mean, I know, but for some stories that's really the only way they're going to get publicity.
"I wanted to continue beyond the end of the source material."
Hmm, better, but still pretty vague. I probably won't read it.
"I wanted to explore [a character's] background and history" or "I wanted to explain something about the source material's universe."
Not bad. Depending on the character and what I see from the first page or so, I might read it.
"I had an idea for a twist or a new angle that I thought would put [a character, story point, an event or whatever] in a new light."
This will definitely get my attention because it's so rare to see. And I'll give any such fanfics at least three chapters to hook me in the rest of the way.
So in conclusion, my feeling is that most fan fiction sucks because most of the authors don't have any good reason for writing the story. They can't even explain their reasons beyond some variation of "I just thought it would be cool, don't you think it would be cool?" But if they've taken the time to think about why they're writing this story, then the chances are pretty good they've taken the time to think about plot, characterization and flow as well.
Incidentally, I have a number of fanfics, but I'm not going to just post a profile and expect you all to come swarming in. Let's face it, you don't really care about the fact that I have an account at FF.net and have published a thing or two. So instead, I'll just present one story. Seems like a lot of people here like anime, so how about a Love Hina fic? It's a one-shot set long after the anime because a fact of life is relationships and feelings change a lot over a long time. I thought it would be interesting to see how the passage of time can change the characters we know.
If you're interested in that junk story, click the link.
http://www.fanfiction.net/s/2647159/1/
Go ahead. Rip it apart. I'll probably thank you for it.