Nothing is at stake, so nothing matters
When I wrote a review of Civil War II a couple weeks ago, I referred to a couple characters as "temporarily dead." This highlights a major problem for comics: Nothing is at stake. Characters die and come back all the time. With characters like The Hulk or Superman, their radiation-enhanced or alien physiology would allow them to come back from catastrophic injuries that should be fatal. But even street-level characters like the Punisher can be brought back, though through convoluted and frankly rather silly means.
This is the problem sort of unique to comic books, which movies and even long-running television shows do not have. A long-running TV show can kill off a character when an actor leaves the franchise, and no one expects a character to last forever. In a movie, the timeline is much more compressed, with the storyline generally being resolved anywhere from 90 and 150 minutes. Sequels extend that, of course, but it is also more likely that actors will opt out of future films in the franchise.
With comics, characters are not tied to an actor's age or willingness to continue playing the part. Batman has been 35 years old since the 1930's, and other characters have been around for decades too. Even characters introduced in the 1990's are a quarter-century old, having started their heroic or villainous careers when they were in their teens or 20's. (When I was in high school, it bothered me that Iron Man was in the Vietnam War and was somehow not an old man.) Therefore, characters that fans have known for decades can continue to be featured - and it is more reasonable for fans to demand they come back from the dead.
The problem with this is obvious: Nothing is at stake, so nothing matters. When I am reading a story, I know that the characters are not in any real danger, that if they are killed they will only be temporarily dead, and that eventually everything will revert to the status quo. So why would I get invested in a story when I know there are no real consequences, for either the heroes or the villains?
Here are two and a half reforms that I would suggest.
Reform #1 -- The death of a major character (or even a minor character) should be an event. When Barry Allen temporarily died in the Crisis on Infinite Earths, he died saving the entire universe. His death paved the way for Wally West (the former Kid Flash) to become the new Flash, a legacy hero that many comic fans actually prefer to Barry Allen. The death of Barry Allen reverberated through the entire DC universe. Similarly, the temporary death of Ultimate Spider-Man was an event that saw the creation of a legacy hero.
Reform #1.5 -- In addition to being a major event with serious repercussions, the death of a character needs to be handled well. The death of Barry Allen was more accepted because of the respectful way the character was handled. Green Lantern Hal Jordan, however, went insane and successfully obliterated all of reality before a few heroes stopped him and restored the universe. Hal Jordan eventually died heroically while saving Earth, but that did not undo the disrespect shown to a character that many fans adored.
Reform #2: Characters need to stay dead. Death is so meaningless that it has become an in-continuity joke. One particularly memorable moment was after the temporary death of Captain Boomerang, one of the police officers at the scene of the crime said something to the effect of "I am sorry your father is dead, but this time he is going to stay that way." If a character is killed, he needs to be dead forever, with no hope of being brought back.
My final advice is that if these two and a half reforms are implemented, not one single character should be killed off for a full decade, to give fans time to breathe. Once that cooling off period is over, deaths of characters should be incredibly rare and should always be permanent.