(A series wherein I propose the sweeping changes necessary to salvage these once-beloved characters)
Jack "King" Kirby - Everything Kirby ever created for DC is back - exactly the way it was when he last wrote it (i.e., if the man himself killed off one of his characters, then they stay dead)*. The goofier, the better! He was a literal creative god who strode upon this earth, and one would have to be a fool to reject his gifts.
That said, the following exceptions do apply:
1. Whereas Neil Gaiman's Sandman would hereafter be considered in-continuity/canon, nothing from Kirby's title of the same name should be allowed to contradict it.
2. The Newsboy Legion could be updated ever-so-slightly to better reflect modern times (for example, gathering intel for internet-based media outlets rather than shilling papers out on the streets).
3. The world of Kamandi, the Last Boy on Earth, and its titular character, is still Earth-51. However, where Countdown got things wrong was in erroneously assuming that there should have been - or ever needed to be - an explanation of the "Great Disaster"/the way things got to be that way in the first place.
I mean, if we all readily accept an Earth where things are (roughly) the opposite of how they are here, with little-to-no explanation given, can't we just accept Kamandi's world as-is?
*EDIT - Exception: Having literally just finished reading Cosmic Odyssey for the first time (12/1/13), I've decided that the events of this miniseries should still stand (the contributions to John Stewart's characterization, for example, are just too important for it not to count).