We have spent a great deal of time discussing the original Crisis on Infinite Earths - discussing the way the event was constructed, it's goals and its frustrations, and its dubious legacy. But the one thing we haven't actually mentioned so far, save for in passing, is the actual story itself. That isn't necessarily because the story itself is secondary: on the contrary, the story - for all the legitimate criticisms that can be levied at its density and self-referentiality - is nevertheless extremely well constructed and quite memorable for those willing to invest the time and patience.
Ask anyone, 25 years after the fact, what exactly happened in Crisis, and even people who have never read the book know exactly what happened: Supergirl and the Flash died. Sure, the multiverse was collapsed, decades worth of subsequent stories were launched, dozens of secondary characters either died or received new status quos - but everyone remembers the one-two punch of Crisis #7 and 8. (The former of which having been responsible for launching one of the most homaged / parodied cover memes of all time.) The Barry Allen Flash and Supergirl are inarguably the two most iconic products of the SIlver Age - the first appearance of the former having actually inaugurated the Age, with the latter still to this day inextricably identified with Camelot-era super-heroic optimism. Their deaths were calculated to shock, and this shock in turn was marshaled for the purpose to sell the significance of the event at hand.
As much as certain segments of the fan population wanted to bemoan their deaths, it's worth remembering that neither Kara nor Barry would have died if they'd been more valuable to the company as living properties than as dead memories. The Flash's own book had been canceled following a long-running and controversial (if fondly remembered by some) storyline that culminated in the Flash being tried for murder and eventually retiring and settling happily in the 30th century. Supergirl trailed a string of inconsequential relaunches and back-up features behind her like confetti. These were popular characters, perhaps, but they were popular in memory - you would have had a hard time convincing most readers in 1985 that these characters' best days were not behind them.
WIth the "death" of the Human Torch fresh in memory, it's interesting to contrast the state of death in comics now as opposed to death in the mid-eighties. Now, of course, it would be absurd to suggest that a prominent character would ever die permanently. Death is a temporary condition at best, and its generally assumed that a good death only makes for a more interesting interregnum between death and resurrection. The best example of this is Captain America. He died about as definitively as a person can die: he was shot to death, his corpse was seen on-panel numerous times, and we received independent verification from experts such as Wolverine and Dr. Strange that the body was, in fact, the real deal. But even given the hoops jumped through by the participating creators to ensure that the death was genuine, there was no reasonable doubt that the resurrection wasn't already on the books before the ink had dried on the death certificate - I am certain that the method of resurrection was vetted before the death ever saw print. That no one bothered to point out that cribbing the plot of Slaughterhouse 5 would be a somewhat unoriginal way to resurrect a character seems to have gone unmentioned. (Interestingly, this was one case where the success of a character's death may have partially backfired on Marvel, with a number of fans stating a very vocal preference for the post-Steve Rogers Captain America status quo.) Everyone knows that the Human Torch is on his way back sooner or later - and, to give credit to the creators involved, there's really not even any pretense that Johnny's death is anything more than a significant wrinkle in a well-planned long-term masterplot.
I don't want to give the impression that death was, in reality, any less porous in the mid-eighties than it is now. Anyone who wants to can Google the phrase "alfred outsider" can see that death has never really been anything more than an inconvenience for marquee characters. But at least in the perception of fans and certain creators, there was a strong feeling that, by the early eighties, death "counted" more than it ever had in the past. Think back to the most famous, era-defining storylines of the period: the Dark Phoenix saga in X-Men, Elektra in Daredevil, "The Judas Contract" in Teen Titans. All these storylines were built around the death of a signature character, and done so with the expectation that that the characters in question would not be subject to an immediate resurrection down the road. (Yes, Jean Grey wasn't initially slated to die in X-Men #137, but once Jim Shooter decided she had to die, Claremont and Byrne committed to the storyline with admirable vigor. It must be said that Jean's death was the catalyst for the book's most fondly-remembered and well-regarded cycle of stories - would the book ever had become Marvel's #1 franchise without "From the Ashes"?) Of course it goes without saying that all of these characters have been resurrected: Jean Grey was brought back just a few years later, in a series of stories for which Byrne was at least partially responsible; Elektra's return was teased by Miller but eventually done without his participation or approval; and various secondary characters named Terra have been bouncing around the DCU for decades. The one significant death from the era that seems to have stuck is, oddly enough, Captain Marvel: although he's come back for periodic cameos, and his return almost happened at the end of Civil War, his death has actually stayed on the books, even after Jason Todd and Barry Allen have both come back. You could make a similar argument that Supergirl's death has never been reversed: there have been many Supergirls in the DC Universe over the last few decades - one of them, the current iteration, actually Supergirl's cousin Kara from Krypton - but none of them have been the Kara Zor-El who died at the hands of the Anti-Monitor. So there is that.
There's no such thing as a "real" death in comics, and there never has been, but the signature deaths in Crisis carried a significant illusion of permanence. The characters weren't chosen at random, and there were convincing reasons why both of them had to die. In the case of Supergirl, Wolfman made a strong argument that Superman's uniqueness had been blunted by successive waves of Kryptonian survivors whose existence measurably lessened Superman's significance. This element was in turn emphasized by Byrne's revamp and, until relatively recently, was considered a positive addition to canon. It wasn't until the mid-aughts that Kara Zor-El actually returned in any capacity, and her death in turn led to the wholly misguided resurrection of a whole city of Kryptonians, and subsequently to one of the most monumentally misguided Superman stories in recent memory. As much fun as it may seem to periodically play Superman against type by placing him in and among a whole mess of other Kryptonians, it becomes that much harder to sell the importance of a character whose whole schtick is his uniqueness. (The ostensible "hook" of the recent "New Krypton" storyline was, of course, that even with hundreds of thousands of Kryptonians flying around, there's still only one with the ethics and compassion of our Superman. But that hook became increasingly muddied as the series bent itself into any number of painful shapes in order to justify the inevitable and obvious genocide of all the remaining Kandorian Kryptonians.)
Barry's death was perhaps a harder sell, and it's worth noting that Wolfman never made any secret of the fact that there he placed an easy "out" for any successive creators who wished to undo the story. Sure enough, when Barry did eventually return over twenty years later, he basically just ran back into the story after having spent the last few years living in the limbo of the Speed Force. (Not that the Speed Force existed in 1985, but I imagine Wolfman nevertheless intended something along those lines all along.) But still: for an event that hinged on the significance of the passing of torches, and the explicit ending of one era of stories in favor of the beginning of another, it was necessary to kill the living avatar of the Silver Age. Wally West became the Flash, and it's at least partly a measure of the success of Barry's death that Wally became the legitimate Flash in a way that, for instance, Kyle Rayner never became the sole legitimate Green Lantern.
The reason these deaths mattered wasn't that they were "permanent," although the illusion of permanence helped sell their significance. The reason these deaths mattered was because both characters were more or less spent by that time. For whatever reason, nobody cared about the Flash and Supergirl anymore. Perversely, the only way to ensure that people cared about these characters again was to kill them. Wolfman & Perez - and everyone on the creative team, and editorial as well - obviously loved these characters, and did them justice the best ways they knew how: by writing the most heroic and heart-rending deaths possible, with both heroes going out in a blaze of glory while saving the universe from total annihilation. No tasteless decapitations or crass mutilations, no rapes, no bullets to the head, no uncharacteristic feuds with other heroes - just courage and sacrifice in the face of impossible odds. Good way to go.