Giving Batman an allegedly biological son, only to have him switch between good, evil, and annoying, before getting blown up on a yacht in the space of only a few issues. (And yes, I know he's coming back soon.)
The Energy Superman of the 90's. Not the fact that he was put in a new costume and rendered powerless, or anything... I just think it could've been done a hell of alot better.
Retconning, reconning, then re-retconning Birthright as Superman's official origin time and time again. For **** sake's, make up your mind, Didio.
Even attempting to touch the Hush character after Loeb's initial arc. They took a character with potential and initially drove him into the ground with crappy arcs and appearances following.
The fear demon crap of Green Lantern: Rebirth. I didn't mind bringing back Hal. Hell, I like the idea of having Hal as a Green Lantern again. But Jesus Christ, don't take away the only freaking thing that made him remotely interesting to do it.
Killing off Azrael. Yeah, he was, at best, a Batman-wannabe. But he was still a hell of alot more interesting than Jason Todd or Stephanie Brown.
Whatever the hell they've been doing with Superman for the past, what, year?
"Red Arrow". Arsenal was a hell of a better name, and it definately fit Roy alot more.
Bart Allen becoming The Flash. If he hadn't, he would've still been Kid Flash, which was the role he was born to be, and he would've never died pointlessly, and Wally would have never gone through such crap to get the role back.
Martian Manhunter becoming emo. Well... more emo than usual.
The Scarebeast.
Selina Kyle having a baby and giving her role as Catwoman up to Holly. The baby part was actually an interesting storyline, but Selina should still be Catwoman. Or, at the very least, retire the role so no one else could take it rightfully.
Oliver Queen and a highly mischaracterised Dinah Lance getting married.
Dick Grayson and Barbara Gordon
not getting married.
Turning Talia Head into a b****, after a good thirty years being an interesting character torn between the love for her father and Batman.
Turning Renee Montoya into The Question. I don't know whose idea that was, but they must've been smoking something
real good. I mean, The Question having a successor isn't the worst idea ever. Just... there are better characters to do it with. Even Booster Gold is more fit to take Vic's place than Montoya.