Tony Stark replaced by Jim Rhodes
The whole "Demon in a Bottle" storyline was pretty cool. I mean, here was a hero who actually hit the bottom of the barrel and had to give up his suit because of his actions. It was shocking for the time. Unfortunately, I think the "Ultimate" line of Marvel has taken this and made it a caricature of the depth of the story we originally read.
At any rate, Tony lost the armor and Rhodes took it. This is another case of where the replacement never really leaves, as he later became War Machine. Not only that, but he even made it into the second Iron Man movie!
Oliver Queen was a fairly standard fixture in the DC Universe. When he was killed in an explosion, it seemed like the natural successor to the title would be Roy Harper, a.k.a. Speedy/Arsenal. But DC pulled a fast one on everyone by bringing in Green Arrow's son Connor to take over the role.
Connor was every bit as good an archer as his father, though his hand-to-hand combat skills were marginally better. He also had this whole pacifist-type thing going on, as a direct opposite to Oliver's more aggressive style of heroing. The coolest part of this replacement was when the new Green Arrow teamed with the new Green Lantern, which allowed for a really neat "Together again for the first time!" situation.
Eventually, Oliver came back and now Connor is dead. The more things change...
Ted Kord made the best he could of the situation he was in. He was sort of like an uncool version of Batman...up until the point where he took a bullet to the skull from Maxwell Lord. DC made him cool just long enough to kill him (what we will forever call "The Question Syndrome").
When the time comes for there to be another Blue Beetle, what happens? The scarab suddenly decides it can give super-powers to someone and chooses a teenager to do it. If anyone deserved to be ticked off in the afterlife, it's Kord. All these years of doing it himself with nothing more than skill, and then this stupid kid gets super powers that he could have had all along if the scarab had just given them to him? Unfair!