Fandom can be fickle and whiny and all around annoying. But their largest crime is their unwillingness to accept new characters simply because they feel threatened by them. I call this Martha Jones Syndrome simply because the Doctor Who fandom was the first place I encountered it, but I am sure she was not the first to suffer at the hand of bitter little romantics.
Martha Jones of Doctor Who. Crimes committed against fandom: Not being Rose Tyler, having a crush on the Doctor, being smarter than Rose Tyler, not respecting the memory of Rose Tyler, threatening Rose/Ten as a ship. I understand that fans were mad at Billie for leaving but that is hardly Martha or Freema's fault. Davis didn't pretend Rose had never existed and Martha filled a completely different role in the Doctor's life. Haters just fail to see the big picture.
Ilana Verdansky of LOST. Crimes committed against fandom: Being yet another out-of-the-blue character, taking up last season time, being too much like Ana Lucia (?). This one just baffles me. Ilana is NOTHING like Ana Lucia... I mean, she's brunette? She likes carrying guns? So is Kate if you haven't noticed. I'll agree that her character was unnecessary but lets blame Darlton and not laugh when her head gets blown off, kay?
Monica Reyes of The X-Files. Crimes committed against fandom: Not being Mulder, shaking up the structure of the X-Files, having a romance outside of Mulder/Scully, being happy. There are quite a few people who dislike Doggett but the amount of hate for Monica is outlandish. You think fans would be happy for another open minded character because Scully and Doggett didn't get much done. You'd think fans would be happy she is a straight female who has no designs on Scully (or Mulder). Obviously that is not enough.
Jane Seever of Life. Crimes committed against fandom: Coming out of nowhere and being awesome. I would have thought that the Life fandom was mature enough to respect a new character who came to save the day while Sarah Shahi was on maternity leave. I mean, what are you gonna do? But Seever was given the same bitter resistance. This is just a massive mystery to me.
Now Allison Mack is pulling a Duchovny and leaving Smallville. I think that fans need to be happy that actors put up with being tied to tv shows for so long and be grateful for what they get. BUT THAT'S JUST ME. TRYIN' TO BE NICE. Whatever.