Hi! I've been around fanfiction.net for 5 years now before I made an account. I'm just a reader, and am unlikely to post any fanfics. If I ever do, they'll be short.
I mostly keep to the Digimon and Kingdom Hearts fandoms. If a "Flightfoot" has ever reviewed a story in either of those fandoms, then that was probably me.
Since I've read so many fanfics, I've found some ways to narrow down massive amounts of stories to smaller amounts of great stories.
Note: These are just guidelines for getting a smaller number of fics and a larger percentage of good fics. It cuts out a lot of good fics with the bad ones. It should mostly be used when going to a new fandom and looking for the best that fandom has to offer.
1. Set the ratings filter to K-T.
2. Change the word count filter to a fairly high word count.
There are plenty of good fics that are short, but the best ones, the ones that I remember as being fantastic stories, are generally over 40,000 words. It takes quite a bit of dedication to write that much, and people who are that dedicated tend to write good fanfics.
3. Set the status to "Complete"
There are many great stories that are in-progress or incomplete, but generally, if you have a choice, you want to be able to read all of a great fic right then. This is more to block the incomplete (and always will be) fics than the in-progress fics.
4. Examine the summary for mistakes
If a summary has spelling mistakes, grammar mistakes, or is just difficult to read, then the fic will probably have those same mistakes.
5. Look for a fic with a large number of faves relative to the standard of that fandom.
Different fandoms have different standards of what is "a lot". In small fandoms, 10 faves may be a lot, but in larger fandoms, like the Naruto fandom, 10 faves may be paltry.
6. If a fic is labeled "Romance" and has two main characters listed, then that's the main pairing that the fic is focused on, unless stated otherwise. Look accordingly.
This one ought to be obvious to anyone who's been around the site for at least a few days.
( The site went through an overhaul since I wrote this. Now fanfictions may have more than two characters listed. Character pairings are indicated by the character names that are inside brackets. If no characters listed inside brackets, then the fic probably doesn't focus on a main pairing, unless it says otherwise.)
For one reason or another, you may want to find bad fanfictions. I've got some tips for finding those, too.
1. Set the filter to "M".
Fanfics that have to be rated M have a high chance of being really bad and disturbing.
2. Change the word count to a pretty low word count, say, 5000 words or less
Most bad fanfics are pretty short. Only a handful make it past the first chapter. There are occasional horrible fanfics that make it past 5000 words, but they are the exception, not the rule.
3. Look for unusual pairings.
As most FF.net users know, if you want to find a pairing, search for the two characters in the pairing in the "Romance" genre. For some unusual pairings, you may only need to search for one of the characters. For instance, when searching through the Kingdom Hearts Fanfiction, pretty much any character pairing with Tron would count as "unusual".
4. Examine the summary for mistakes and for general badness.
If the summary is really bad, than the fanfic probably is as well.
5. Search for "first fanfic", "crack", "smut", "OC", and/or "OOC".
If the fanfic is the author's first, then it's more likely to be bad, since the author doesn't have much experience.
Crack fics are usually utterly ridiculous and are often intentionally bad.
Smut fics are basically porn, so they usually don't have much in the way of plot, characterization, good grammar, or general decency.
Good OC fics are hard to write, so naturally, a lot are pretty bad.
If even the author thinks that the characters are OOC (Out-of-character), then you can be pretty sure that they are.