Basically if you have to alter the characters personality or can't keep them in-character for a fanfic, that pretty much means you shouldn't attempt it (lol) or something like that. At least make the pairing believable in the way that the characters are portrayed in the series. There is "flexible" writing, then there is just "OOC", "my version of (so-n-so)" or "how it should be but the creator is too stupid to see it - so naaay!!".
Though I don't seem to have an issue if people mention OOC ahead of time. I don't have a probably with fanart of the characters paired together, though the artist comments can throw the image off a bit. Obvious no one will ever single-handedly stop pairings they don't like seeing (even the creators themselves). And it is always amusing to see who people match characters up with.
__NEGIMA!
Negi / (anyone)
The kid has no interests in a intimate relationship with females or males, and due to him being a child his mind isn't getting the obvious vibes from females that outright said they love him. With the variety of characters, I can see why people try to match him up by some means. I just personally prefer Negi to be carefree and pair...less.
__A:TLA
Zuko / Katara
Because it was never even plausible... she never liked him through half the series, the other time she just "got to know him", and then they become some kind of frienRAB. Other than that neither has romantic feelings for the other at all. Plus to me, it just doesn't work, even in fanfiction.
__Death Note
L / Light
Overall, it is just one of those series were pairing just doesn't work very well.