1.) Not using spelling/grammar check. There is absolutely no excuse for this. Nobody wants to read a story wrought with typos. It distracts the reader's attention from the body of the story, and alerts them that you haven't put a great deal of time and/or effort into your work.

2.) rIting iN l33t 5p33k, im lingo, or just real bad enlgish. Instant Messenger did not exist in 19th century France, when Les Miserables took place. Nor did "leet". As such, it is completely inappropriate to use words/phrases such as "like", "omg", "2", "u", "ur", ad nauseum. If you're going to post a fic on the internet, at least do your readers the consideration of typing out "your" or "too". It ain't that difficult, kiddies.

3.) Anachronism. This one *can* frequently go unnoticed, if only in its smallest, most unobtrusive form (say, a faulty historical timeline). However, if Enjolras is wearing Levis, Cosette and Eponine are using phrases such as "okay", "yeah", and "omigosh", and Bossuet's upset because his phone's been shut off, and it's NOT a parody, there's a problem. If you aren't sure about a significant historical detail, LOOK IT UP. Not doing so will destroy your credibility as an author.

4.) Mary-Sues. Everyone's guilty of having written one, at least. I have done it myself. Luckily, I discovered that there was a name for this fanfic evil before I began the descent into the world of OFCs. Don't misunderstand, when done correctly original characters can be delightful, and there are many that are enjoyed by the fanfiction community at large. However, for every one that's charming and tolerable, there are three that are completely obnoxious. For more information, I recommend The Mary-Sue Society.

5.) OOC-ness. For anyone who knows their canon, the following scenarios should be red flags:

-Enjolras keeping a mistress.
-Eponine having knowledge of dental hygiene.
-Cosette being a psychotic raving bitch.
-Thenardier dressing up like Santa and buying Christmas presents for all the poor children of France.

Some OOC-ness is deliberate (witness the many wonderfully written and enjoyed slashfics out there). The difficulty lies in an author having no knowledge of canon, and thus seeing nothing wrong with playing matchmaker to Enjolras and Patria (who really *is* a devastatingly beautiful, revolutionary female lawyer, and not a metaphor). This can be avoided by reading The Brick, or, alternatively, getting a beta-reader with adequate knowledge of canon.

5 1/2.) Out of control AU-ness. A fic in which Enjolras survives the barricade, while cliche, has the potential to be entertaining. A fic in which Eponine survives, marries Marius, all of Les Amis live and start a new republic, Javert and Valjean finally confront each other and settle their differences with a firm handshake, and Grantaire stops drinking and reforms himself, is bound to go over badly. If this scenario is familiar to you, consider getting a beta-reader to reign you in when you start going wild.

6.) Self-insertion. While I would dearly love to write myself into Les Miserables, befriend all of Les Amis, join the revolution, save Enjolras from certain death, blah blah blah...I won't. This is a big fanfiction no-no.  Myself, I'm sure the Fearless Leader looks just spiffy with his shirt off. However, I have no pressing need to write about it and post it online. At best, nobody cares to read these kinds of fics. At worst, people will rip them apart.

7.) Angsty poetry and/or songfic. A poem or song about Eponine in the spirit of "Marius doesn't love me, even though I *am* really a good-hearted, generous, delightful person, and not a lunatic, stalker, harlot, and attempted murderer" is ill-advised. It's been done. To death. The rare few with a genuine, innate talent for poetry have managed to pull this off successfully. The rest of the poetry in this genre is typically a dime a dozen. Instead of tackling well-worn plotlines, try to invent some of your own. Use unique combinations of characters (i.e. Azelma and Javert, or Enjolras and Mme. Thenardier). The possibilites are endless.