My opinions on good and bad fanfic are gradually becoming slightly less black & white. I've been reading a *ahem* fair amount of fic (an insane lot, actually, all my spare time for the last month has gone into reading teh smut) so let's consider this an informed opinion.
I've become able to consider writing, plot, character and perspective/idea as seperate elements. I've found well written fic that bore me, not so well written fic that intrigue me and otherwise unremarkable fic that have these aspects that tickle me nonetheless.
I've found that good writing is less important than I thought it would be, up to a point. Meaning, I'm not going to read something that doesn't intrigue me, no matter how inoffensive the writing but I will read something that I find interesting even though the writing sets my teeth on edge. Again, up to a point. The you're/your and their/they're thing kills me. Not if it's a mistake but if it is consistent it ruins my pleasure. I'll never know if that fic was worth reading anyway, I'm not even going to make it past paragraph 1.
Another pet peeve: period inconsitencies are doable but blatant modern day inserts that are glaringly out of place and OOC make me cringe. Like, do not let your FMA characters anywhere near a Starbucks, please. I am begging here and I'm a proud woman, this doesn't come easy ;p.
And! I've found 1 real, actual Mary Sue so far! That was funny but the fic sucked. There is a fine line between good amounts of automail (think Ed) and bad amounts of automail (think Archer). Keep this in mind if you really, really need to be cooler than Ed ;).
With regards to writing: I have to admit to downloading fic and tweaking it a bit... I've changed certain words and phrases that really bothered me so I could enjoy the rest. Now, I've no scruples about correcting typos and stuff but I feel slightly bad about the tweaking. I wonder what authors think of that?