1. TERRIBLE CHARACTERISATION!!!!! My god, people. GW boys are child soldiers, trained to kill. The day that Heero proposesn is the day the world collapses on itself. Characterisation is fucking IMPORTANT! Guys are allowed to be cranky, angsty, violent, and not lovey dovey.
2. HORRIBLE NICKNAMES! This is for you, Spike/Xander people. Please STOP making Spike call Xander horrid, horrid nicknames, like "Xan-pet".
3. GIRL BASHING! Get over it. Chicks rock, and not all of them are there to prevent the hot boys from making out. Unless the girl is a complete bitch in the canon, why make her into the shrew?
4. RANDOM ABUSE! Honestly, if you can't come up with a decent reason for a character's moodiness, why not make them a sufferer of abuse? Some of the best and worst fics I have read have involved abuse of various sorts. At best it is soul wrenching, and painful. At worst, it's a cheap, nasty way to make a character vulnarable.
5.SHIPPER WARS! Oh. my. god. Get the hell over it. I read specific pairings, just like everyone else. Some pairings I think of as stupid. Does that mean that I waste my time having screaming matches over the net? NO. F'rinstance, I really don't like 1x2, or Spike/Angel. Does that mean that I flame and abuse those that do? Hell no. The only time I have ever flamed someone based on pairings was because of character assassination. I regret it now, but I still stand by that.
6. SPELLING AND CHAT SPEAK! If grammer, spelling and sentence construction elude you, do NOT write fanfic of any sort, I implore you. That is all.