Like this post.Doomsday Clock: This was hyped so much and almost nothing came out of it. Geoff Johns gave us back the pre-Crisis and New 52 universes and DC just straight up threw them away. I don't know what this was supposed to accomplish but whatever it was, it didn't. I guarantee none of the concepts created in it will ever be mentioned again.
Infinite Crisis: Again, brings back Earth 2 Superman and reminds everyone of the pre-Crisis universe and just turns Superboy Prime into a villain and kills off Earth 2 Superman and then literally drags his corpse around the DCU until the reboot. Complete waste of time.
Hypertime: This wasn't an event per-se but it was a backdoor way to bring back the multiverse that no one used. Waid played with it around 1998-99 and then it was just kind of forgotten about.
Convergence: There's no doubt this killed the New 52. You don't remind your audience of the previous universe less than five years into the one you're trying to sell to the public and is struggling.
New 52: This had so much potential but Didio just wouldn't get out of his own way and let the people he hired do their jobs. He had to micro-manage everything and a lot of his ideas were just bad. No one allowed to get married. Sidelining Wally and Dick and just kind of hoping people would forget about them. This was a chance at the kind of hard reboot that DC needed but they couldn't pull the trigger or stick with their own ideas. This leads well into the Convergence problem. They wouldn't let go of the previous universe. You had anthology, out of continuity, series like Sensation Comics and Adventures of Superman that kept reminding people of the old universe while you're still trying to sell the public on the new one. After Rebirth there were all kinds of conspiracies that said that New 52 was never meant to be permanent in the first place and it's easy to see why people would think that. It didn't help that any tie in merch, like that video game, tanked also. Just fed into the problem.
COIE: This one's tricky because, for all intents and purposes, it did what it was designed to do. I.E. boost sales. But the longer term consequences are still being felt to this day. One could chalk that up to just mismanagement at the time but we're still correcting for those problems today. I was seven years old when this happened so I'm not privy to what the comics market was like at that time. But given the fact that there was serious consideration to just closing up shop and letting Marvel publish their books, it sounds like things were dire. So this may have been the best decision financially. But looking at the books at the time and leading up to it, it also looks like more could have been done to avoid it and just wasn't.
I haven't followed a whole lot of "Crisis" events since DDC but by the sound of it, none of them really had any lasting impact.
Almost all events start strong and end weak!
COIE is the exception...and the mess it made haunts us to this day.