1.) It's been done before. A lot. And I'm not just talking about Superman's adoptive father Johnathan "Pa" Kent, although he does seem to be in a near-constant state of being killed (By the way, friend of the site Troy Brownfield wrote a reader-friendly guide to the many deaths of Pa Kent across all Superman media for Newsarama the other day).
No, I mean the killing off of a character to provoke a reaction from another character in general; DC has played the "Oh no, _____ has died!" card so many times of late that it's hard to beleive even the most hardcore DC fans have any sort of emotional reaction at all. Sometimes it's another hero or a villain (who will immediately be replaced by a legacy version) and sometimes it's a supporting cast member, either one just introduced or one relatively long-lived, but DC has been attacking their character catalogue—by far their most valuable resource—with a threshing machine for longer than I've been blogging now.
I realize it's unfair to say that all DC Universe stories are either about a character getting killed, coming back to life, or the universe's continuity being somehow reset, but it sure seems like it, doesn't it?
2.) It's not like anyone's actually going to do anything with it anyway. I suppose the excuse given for killing Pa Kent—probably being proferred in interviews I'm not reading that have recently been posted all over the Internet—would be/is that it will allow Superman's current writers to explore Superman's soft, vulnerable, human side, to elaborate on the fact that there are some things even Superman can't do, and tell stories about the emotional toll of death.
But honestly, what are the chances of that happening? Superman's writers and editors are already talking about the next big Superman story, which involves the shrunken bottle city of Kandor being regrown on Earth, filling the world with more Supermen then ever before. Will that epic adventure be interspersed with scenes of Clark Kent crying at his dad's funeral and trying to convince his widowed mother that she should get an apartment in Metropolis now? Does anyone really want to read that story?
While DC tends to do these big, status quo-changing stunt stories a lot, they rarely do much in the way of follow-up. When Tim "Robin III" Drake's father was murdered in the pages of Identity Crisis, the Robin monthly was focused on the character fighting super-assassins hired by The Penguin for months, followed Teen Titans and Shadowpact crossovers. Batman has seemed extremely unconcerned to find out that his former sidekick Jason "Robin II" Todd has returned from the dead as a murderous vigilante, that his former sidekick Cassandra "Batgirl II" Cain suddenly became an evil murderer and the head of a guild of assassins or that there was a lady calling herself Batwoman running around Gotham City fighting crime while he was on vacation; he's been busy catching Ra's al Ghul, Mr. Zsasz and The Scarecrow for the 90th, 15th and 300th time (respectively) instead of being featured in stories using reactions to such gamechanging events to develop his character in some way.
Most relevant to the Superman books, Geoff Johns and his co-writer Richard Donner introduced a Kryptonian boy named Chris Kent that Superman and Lois adopted, a character that starred in a single story arc of Johns and Donner's (and at least a half-dozen of Kurt Busiek's) and then just completely disappeared, never to be mentioned by any of the characters, as it would only draw attention to the fact that Action Comics wasn't edited to be part of the cohesive whole of the DCU, apparently because a "famous" person from a different field of media was involved (Without getting into it too much, all of the Busiek stories were set after the Johns/Donner one, but since Chris disappears at the end of the Johns/Donner arc, the stories can't logically co-exist).
So I sincerely doubt we're going to get any powerful tales of mourning and loss in the future; instead, Superman will simply list the death in his catalog of things that make him sad, thinking to himself "I lost my birth parents, I lost Krypton, and I lost Pa, but I refuse to lose you too Lois!" as he swoops in to save her from Metallo or whatever.
3.) The fact that Superman's parents were still alive and that he had a normal, healthy relationship with them was one of the things that made him unique. Well, at least unique among comic book superheroes. Hell, I'm hard-pressed to think of another major superhero at either DC or Marvel that had both of their parents still alive, married and always glad to see him or her.
Certainly Superman's relationship with his parents isn't a core part of what makes the character interesting or compelling, and it's not like DC has "ruined" him by removing a single supporting character from the cast (more fundamental "damage" was probably done when they married him off to Lois, and yet he survived that and still works quite well). But, as a reader, it's something I liked; something that separated Superman from Batman, Wonder Woman, Captain America, Spider-Man, Aquaman, all the Flashes and Green Lanterns and so on.
That sort of relationship was one of the things I found most appealing about the new Blue Beetle series; the way Keith Giffen and John Rogers just dispensed with all the hoary secret identity cliches and gave their young hero a supporting family who were not only a large part of who he was, but always had his back. (Blue Beetle still gives readers that sort of hero/parent relationship then, but Blue Beetle is no Superman, and the book always seems to be hovering near cancellation level, whereas I think it's safe to assume that as long as superhero comics are being published, Superman comics will be published).
4.) It diminishes the character. Superman's own father died, and he couldn't save him? That's not very super, is it? I mean, the fact that he can do anything is kind of the point of Superman. This isn't a naive young Superboy still coming to grips with his emerging powers or anything; this is Superman at the dizzying, god-like height of his powers.
This is a guy who wrestles angels, contends with devils, and beats-up evil gods and the characters of classical myth on a semi-regular basis.
He's outraced death itself, returned from the dead (repeatedly), has visited heaven and hell and all points in between, and defeats an opponent from a higher-dimension every three months. This is a man who knows every time-traveler, wizard and miracle-worker on Earth—and quite a few off of the earth—on a first name basis.
In Superman Beyond, another book occurring around the same time as Action Comics, he was keeping his dying wife's heart beating using his vision powers and was able to lift a book with an infinite number of pages. His alternate reality dopplegangers from Earth-2 and Earth-Prime are strong enough to break walls between universes and bend the nature of reality with their bare hands. His All-Star self is practically, if not literally, God.
If he can't save Pa Kent, he either doesn't much care for him or isn't trying hard enough.