I remember Armageddon 2001. I remember feeling ripped off, since it made no sense (outside a marketing standpoint) for Waverider to check the futures of some heroes (mainly Superman and Batman) multiple times. And then the baddie was some relatively minor hero that he'd already checked out.
How about the DC Annual crossover ("New Blood"?) where aliens came to earth, started drinking blood or something, and inadvertently created one new hero per annual. . . each of which promptly faded away.
Or the worst move DC ever made, IMHO: making Hal Jordan, the Green Lantern, go wacko for no real reason and kill the rest of the Corps, only to be replaced by a knockoff in a snazzier costume? Up until Emerald Twilight, GL was the best book DC published. After that point, meh.
And to keep them alive when favorites like Kilowog were offed, DC made Jon "Not the Cooler Guy Who Hosts The Daily Show" Stewart one of those GL ripoff Darkstars, and turned Guy Gardner into some freaky-deaky alien mutant piece of crap thingy.