I will have to agree with Jodan on self-inserts. Sure, you can fantasize about having sex with a made-up character, write fanfics about it and all that, but I don't see why you should post it for everyone to read. We don't even know you, why would we be interested in reading about you getting it on with an anime character?
And I should add that "original characters" and "self-inserts" are completely different things.
Let's see, things I hate...
Mary Sue/Marty Stu (any time, but ESPECIALLY in lemons)
Character bashing of ANY kind (ESP of the "[character 1] loves [character 2] but [character 2] is fucking [character 3], so [character 1] rapes [character 2] and is lynched by the rest of the group"variety)
People claiming that their favourite couple is show canon (not just lemons, at any time)
I normally have no problem with scriptform, but it just doesn't work for lemons.
Inexplicable, unexplained gender changes (this means you, Patamon people.)
Ditto unexplained incest (hypocrite, you cry reading Prophecy {I'm Pistoffveemon, BTW} not so, I reply, pointing out that there sure as hell was a reason for that)
Ditto anything with a completely new cast
I'm sure there's more things I hate, I just can't bother to think of them now.
Here's one I forgot and wish I never had to see again.
5.) A child's adult fict.
Disclaimer:
'This fict for adult only. No child. Contains sex.'
Extent of description in fict:
'Kari n' tk made love and then she was going to have a baby!'
6.) Evil Incarnate
Using example from number 5: 'Davis got jealous and killed the wonderful kari.'
Where little explanation ('Kari luvs TK'), a 'former hero' becomes a psychopath that would give Satan the creeps. And why? Because the evil jerk/bitch is trying to break up the 'perfect' couple/steal MY beloved. I mean, how DARE Jun try to make time with Yamato when that's what _I_ want to do. So, I'll show everyone how evil she is.
7.) I'm more than the author, I'm also Mary Poppins and I've become the lover of my favorite character.
I've seen self-insertions ficts done well, where the author kept themself human. (I went to the Tendo dojo and quickly realized that I don't speak Japanese and I couldn't fight my way out of a wet paper bag-- ouch.)
However, this point is for the 'partically perfect in every way' insertions where everyone loves you and you either have the most perfect life or your past makes Harry Potter's look like paradise. And no matter how much the favored character is shown to not like something. Let's take say, oh, Yamato's aversion to highly extroverted girls for example but miss perfect extrovert on speed, guana and sugar buzz has Yama-chan wrap around her little pinky at 'hello.'
...AHAHAHA!! I was a jackass!!...Oh man...And usually if I sense something doesn't feel right after the first two paragraphs...You know...That feeling you get...Then it ain't right...Or at least something isn't...