I don't think I've ever said "Oh, boy, a retcon!" with any sort of enthusiasm, especially if adds something new to a well-established and well-loved story.
Fleshing out information between panels or taking a short story and adapting it for a modern audience is one thing; for instance, the first volume of Ultimate Spider-Man remains true to the 1962 origin even while expanding on it in a contemporary way. I'm talking about the addition of a new character who was also there at some momentous time, or a "dark secret" a character has been keeping for years and we only now learn it affected everything we've been reading.
I think those kinds of retcons are used mostly to make a current storyline or character more important by connecting them to the things we already know and love. It's even worse when they don't stick around and are quickly forgotten under a new creative team. I'm still a little sore at Ed Brubaker for X-Men: Deadly Genesis (for reasons that would take up the rest of this column), and I'm not all that thrilled to see Angela take her place as Thor's sister.
Angela is an Image Comics character: From her design to her origins, she looks and feels as if she's from a different place and time. The signature ever-waving ribbon around her body and that big miniskirt/belt appear out of place in Marvel's more modern costuming styles. Instead of placing her within the context of Asgard, vague elements of her backstory have been stapled into the World Tree as a mysterious Tenth Kingdom called "Heaven," although there's no word on if it's connected to the Celestial "Heaven" or if this is a separate chapter of Asgardian mythology. If it's taking the Christian concept of Heaven and angels and adding it into Thor's mythology, it seems almost disrespectful to the source material ... but well within the bounds of actual living mythology.
Marvel's Thor is patterned on Norse mythology but he's obviously not a direct copy; the actual mythology doesn't have any spaceships, for one. By now, the ideas from the original mythology have been adapted by Stan Lee, Jack Kirby and scores of others to represent their own interpretations of more modern views and styles of storytelling. Which is what Norse mythology was; as the Romans began to influence Germanic tribes, what we know as Norse myth moved north and became its own distinct form of the original Germanic mythology. Ancient myths were incredibly mutable, as there were no continuity police or Internet forums to complain about retcons. Obviously, a lot of Roman myth was lifted from the Greeks and other sources, and changed by region as the Roman Empire conquered different cultures. It was later rewritten by Victorian nationalists and then shortened by Bulfinch as one man became the standard of classical mythology for nearly a century.
Despite all the elements changed, added, adapted, censored and cut, we still have the gist of what people told one another ages ago. So no matter what Dan Slott does with Peter Parker or how much you may hate "One More Day," Spider-Man essentially remains the same and can be passed on to future generations. Perhaps they'll discover the lost issues of the Mary Jane marriage and add it back into their collective myth?
Angela is an Image character. Sure, she's Neil Gaiman's character, but her design and origin have a lot of that early-Image feel, now being brought into the Marvel Universe. The ancient Sabine goddess Vacuna was originally just a local goddess before she was brought to Rome and connected with other goddesses in the pantheon. Despite hanging out with the likes of Minerva, Nike and Diana, goddesses better known to the modern world, we don't know much about her purpose; while Angela hung out with the likes of the Guardians of the Galaxy and has dramatic ties to Asgard, she still remains a mystery as to what she's doing here.
Her original book, Spawn, has her as an Angelic bounty hunter, tasked with bringing in the hell-bound Spawn; for her Marvel origins, we know she's an Angelic "assassin"/debt collector who works within a strict balance of exchange as nothing is ever given without taking in return. The motives are a little different but the essential elements remain. While traveling along the Rhine, Roman soldiers would erect statues of Hercules with regalia from Germanic myth for the civilian population, as they wanted to spread Roman culture to the people in forms they would recognize. So while the Roman troops would see Hercules with a bunch of stuff, the Germanic tribes would see their own god Donar dressed in a Roman fashion. Up north, away from Roman influence, Donar would come to be known as Thor.
So rewriting mythology is really no big deal, and if we think of modern comics as our own form of storytelling myth and legend, retcons aren't all that out of place. Archaeologists have yet to find ancient Roman message boards complaining about changes in Latin. As we walk along the Rhine of the Marvel Universe, Spawn and Image fans can see the popular character Angela now garbed in modern Marvel storytelling. Thor fans can see a new character in Asgardian trappings, making her more recognizable and easier to understand than slogging through a bunch of Spawn back issues. Both interpretations are correct and hopefully the character will flourish in tales to come.